This post summarizes some of my remarks and observations from different threads, so much of the info presented here may naturally appear somewhat redundant. I am 100% sure there are better Sednas and QoTs than me, but the scarcity of dedicated support players that can actually re-think, analyze and formulate things, as well as me actually getting pretty good with both characters (at least, according to half a dozen top-tier players I play with/against on a relatively regular basis -- maybe they are just nice to me, dunno) can somewhat justify me, and not one of the better folks, posting this... I hope 
I. Sedna
My major issues with current pro builds are: (1) disproportional prominence of Silence; (2) too much emphasis on leveling up Heal; (3) Pounce getting neglected/postponed (at least, in Hedgie's and Hedgie-inspired builds); (4) IG being considered "a selfish skill" and "a newb trap". In particular:
(1) Since practice > theory, and I always get Silence I at 6-8 in skilled matches, I'm revising the statement I made in my previous Sedna-related post Silence I is a must-have at levels 6-8, while Silence II and III aren't a priority and should be postponed until very late game.
(2) Heal should never be a priority after the very first point. Heal I is a must-have early game for emergencies, Heal III is nice to have afterwards due to debuff removal, but I wouldn't call it crucial. Sedna's main source of healing should be HW-enhanced bishops, not her own Heals. Keeping Heal level minimal until mid game also ensures you can always help a focused teammate when needed. +600 hp may not sound like a big deal, but it's an emergency-only skill and it is absolutely adequate to its purpose: preventing early kills by the opposing team.
(3) To quote Hedgie, Pounce is "one of the worst offensive abilities in the game. It has no debuffs, no AoE, you can't use it while you are chasing". On the other hand, it is a fast interrupt with a great cooldown, a decent range (9) and damage, plus you can (and, IMHO, should!) max it out qute early. Its most important advantage is its being Sedna's only decent source of damage, and being able to deal some concentrated damage is a critical quality in skilled 3v3 matches.
(4) I am firmly convinced IG I is a critical early skill, unless you don't level Pounce and have no intention to actively assist your teammates in getting some kills. Besides a great +10 hps bonus, it gives you a +0.3 y/s, which is just enough to keep up with Erbs and UBs early game. Chase and AA until down to 600-800 hp > Pounce II/III got me a lot of early kills. No, not against noobs. Even experienced people tend not to expect concentrated damage from Sedna, especially at level 6-7. That said, IG II and III are selfish skills and newb traps, and should either be postponed or never taken.
Favor: BotF
1. Heal I
2. HW I
3. IG I
4. Pounce I
5. HW II
6. Pounce II
7. Pounce III
8. Silence I
9. CH (generals)/Heal II (no generals)
10. Pounce IV
11. Heal II/Heal III
12. Heal III/MP I
13. MP I/MP II
14. MP II/Heal IV
15. MP III
16. Silence II
17. Silence III
18. Life's Child
19. IG II
20. IG III
Items: 1 helm (Vlemish > Hungarling, if possible), 4 hp items (3 armors + Unbreakables). Sedna is not about skill-spamming. You spam only Pounce. Silence and Heal are saved for specific circumstances. Start with Monks, then at WR3 Cur1 (possibly also FS1) and Banded, then, ASAP, Bishops.
Priority order: (1) Banded; (2) Nimoth; (3) Bishops/High Priests (another general on your team); (4) Unbreakables; (5) Hauberk; (6) Vlemish; (7) Banded > Girdle of Giants; (8) Siege Demolishers; (9) upgrades (Nimoth > Groffling, Vlemish > Hungarling, etc.). Yes, may sound a bit radical, but that's what works best for this build. You don't need a lot of mana. You don't spam anything, all your skills are strictly situational (Pounce is a situational finisher/interrupt skill, not a fire-at-will nuke). Your strongest levels are probably 6-10. You are going to score a handful of kills, if you use Pounce correctly. In a skilled team, the allies will burden you only with Cur1, so you will be able to get the Girdle relatively early. The general principle in DG is: buy early, don't save. However, I believe early Girdle makes this build at least twice as effective: immense hp/hps boost, great help at farming due to 100% Cleave property, significant dps increase for the already heavy-hitting DG. Save instead of upgrading to Groffling+Hungarling and of buying Demolishers ASAP, it will pay off.
If you are sceptical as to whether it is practically possible, a bit of elementary math. Usually, I have Groffling, Hungarling, Siege Dems by later mid/early late game. Banded > Groffling = 4760 g; Vlemish > Hungarling = 4100 g ; Siege Dems = 2050 g. Grand total = 10910 g. Banded > Girdle of Giants = 12560 g. You need only 1650 g more if you don't upgrade and don't get Dems ASAP.
Orb of Defiance is not a bad option, as it's very strong on Sed (less than 1/3 hp left>Orb>Heal with Life's Child active is enough to get those gank-happy DAs frustrated). However, I don't think it's worth it, in a long run. Having 3 consumable slots is more important than an additional safety mechanism and a +500 passive bonus to health and armor: you are already safe and not easily killable, while dropping locks (or, worse, sigils) for OoD is usually a short-sighted decision. If you do get OoD, be sure to take Life's Child at 16: while not a super-great skill, it's actually very good with OoD.
I almost never take Nature's Reckoning. In 3v3, some hp stacking will be a much wiser investment, generally speaking. See section III below for additional details.
Artifacts: Unfortunately, only passive effect (non-useable) artifacts are really viable for Sedna in a skilled match. Having locks, tp scrolls and sigils should always take precedence over everything else. That really limits Sedna's possibilities as far as artifact choice is concerned, in the priority order: (1) Girdle of Giants is the best passive artifact option: get it ASAP, if the game is going well for your team, and for you in particular, you can get it mid-game, if you don't upgrade armor pieces and helm (i. e., don't go Vlemish > Hungaling, Banded > Groffling, etc.) and don't get Siege Dems; (2) Stormbringer (cooldowns halved with a flag + MP III: -47%) is your artifact helm that has a great synergy to MP III; (3) Bulwark: an universal staple, so it's a viable option. However, I don't think it's a high priority artifact for Sedna. If you have 16k and want to get a second artifact, wait a bit and get Stormbringer.
If you are willing to risk dropping sigils or locks, you should bear in mind the main purpose of getting artifacts for Sedna is to compensate for her two main weaknesses late game: lack of AoE and poor AA speed. Thus, you have two good cheap options. (1) Orb of Veiled Storms. Cheap, good AA speed boost, gets late game hp regen through the roof, provides a decent anti-creep AoE (Orb > Heal IV = 700 damage in 10 y radius, enough to deal with a creepwave, save giants). (2) Cloak of Flames. Costs 2k more, but both AoE and AA speed boost are better. Also solves potential late game mana problems that might arise in long skirmishes with 1 helm only.
Role: This Sedna build excels both at tanking/aura buffing and at setting up/securing kills. Of course, since Sedna's tankability relies exclusively on Bishops, they should be prioritized. This Sedna fares well against any matchup, with the obvious exception of Rook (against whom, I believe, she's also significantly better than Silence-Heal Sedna). In short, it's as good a compromise as I could find between strong Sedna ("Fight, goddamit! You do no damage, dude!") and smart Sedna ("Silence that freaking Oak already, dude!"/"Heal, heal, heal, dude! I just wanted to get the kill, so I followed their Reg to the crystal!"). Everyone will be happy with your performance, with the important exception of your enemies. You'll be able to support your main carry and get (steal!) a solid number of kills yourself. Doggu will love you, Kira will love you, Nomilarac will love you, Renz will love you. Hell, maybe even Nnnils will love you, and that's a bold claim to make! Only Pacov will hate you, but that goes without saying.
Levels 1-5: You are quite strong and can afford to take some risks. Most opponents will think they can take on you 1v1, try to use it to your advantage and yellow deny them some flags by big-ass outtanking. What I like to do early game to maximize Sed's tanking potential is to get Monks and run to contest mid ASAP. Regardless of the outcome, split your Monks between hp and mana lanes, and run back to the shop. You should have ~550 g by this point, so buy Banded, then go hp (or mana, if needed). You lose a tiny bit of experience this way, but gain a lot of staying power, so it's a good trade-off.
Levels 6-10: If you play your cards right, you'll get several kills here (800 damage at level 7 is not something people will expect from a support DG). However, be aware you are quite fragile (gimped from purchasing Cur1, and possibly also Cur2 and FS1). If your teammates are aware of game dynamics, they will protect you, so you are in no great danger. Still, I suggest always carrying a sigil starting with level 6, when you start chasing and finishing.
Levels 11-20: Your kit should be complete by level 11, so no need to be overly careful, you can tank if needed. Try to stick to your main carry's arse, especially if he goes deep into enemy's territory. Needless to say, you are the main locker.
II. QoT
My major issues with current pro builds are: (1) unnecessarily Mulch-oriented at early levels; (2) too little priority assigned to SW by most/some super-pro players getting SW III; (3) the basic concept of QoT as main carry is flawed. In detail:
(1) Mulch I is a costly (700 mana, 2 skill points), slow (cast times, summon appearance delay), inconvenient (mad clicking obligatory) way to heal 750 hp. It should be taken only as a pre-requisite to Mulch II and III, which are, indeed, great skills. Getting it at 5 (and investing another point in Summon Shambler!) is a mistake certain top-tier players manage to get away with only due to their exceptional personal skill. It's highly inefficient, and definitely not worth getting, unless you like a challenge. Period.
(2) Spike Wave I and II are great and should be taken ASAP (at 5 and 10). On the other hand, Spike Wave III should never be taken due to whopping 50% cooldown increase.
(3) There is little doubt that in a skilled player's hands, QoT can function as a main carry/tank. However, it's evident it is not her optimal role. Obviously, currently fashionable Mulch-happy pro QoT builds are heavily lure/kill-oriented. Well, what can I say... I'm sure it's possible to drive a nail with, say, an electronic microscope, but I'd rather use a 
, thank you. I don't think gimping oneself as a support for what essentially is a one-trick pony (luring, then getting a kill due to early Mulch) is a fortunate idea. If you can pull it off -- more power to you, but presenting it as the greatest QoT playstyle ever is a bit of a stretch.
Favor: BotF
1. BS I
2. GS I
3. SAVE
4. GS II
5. SW I
6. SAVE
7. BS II + GS III (this is the point you should have enough mana regen to consistently cast 560 mana BS)
8. SAVE
9. SAVE
10. BS III + BS IV + GS IV + SW II (at this point, you should maximize your BS potential, as alpha nukes damage potential peaks here)
11. SAVE
12. SAVE
13. SAVE
14. SAVE
15. Entourage I-III + Tribute + GoT / Shambler I + Mulch I-III + GoT (go the second route only if you feel you need to tank a lot late game: an undesirable scenario, but possible)
16-20: Get Shamblers I-IV (or II-IV + Entourage I) in any case, either for a hefty increasy of mostly anti-structure dps or for easier Mulching.
Items: 2 helms (Vlemish, then Vlemish + Plenor, then Hungarling + Vlemish), 3 hp items (2 armors + Unbreakables). Start with Monks, then at WR3 Cur1 (possibly also FS1) and Vlemish, then Unbreakables.
Getting Magus Rod instead of Plenor is an interesting possibility. It gives an additional hp item slot, but is inconvenient both in that it adds additional microing to an already microing-heavy DG, and in that it occupies one consumable slot. Overall, I think it's preferable to run with two helms, but certain circumstances can surely dictate otherwise. For example, if you get Bulwark at level 15+, you probably should drop locks and get Magus Rod instead.
Artifacts: QoT has no particular weaknesses late game, so artifacts should serve to augment her strengths, instead. Bulwark should be your top priority: 40% permanent damage reduction in unpacked form makes late-game QoT insanely powerful, practically invincible. Bracelet of Rage is also an interesting option, but as it takes up a consumable slot, I cannot really recommend it.
Role: Levels 1-10: You get at least Cur1 (sometimes also FS1 and Cur2), so you are quite fragile (though not as fragile as your opponents tend to think) and will be focused a lot. Play it very conservatively. Stay back, shield your main carry/lane partner, ground spike when needed (learn to pull off the GS cancel trick to make the enemies run away, as mana is scarce), do not forget to spike wave the ganked DGs. Assist in pushing the corner towers with GS. Either learn QoT's skills mana cost* by heart ASAP, or write them down: you cannot see BS costs while unpacked, and vice versa.
*) Please note QoT is the only DG to have two serious skill cost:efficiency ratio progression aberrations. Most skills are somewhat linear in this respect, e. g. Shield I is 400 mana, Shield II is 500 mana, Shield III is 600 mana, and Shield IV is 700 mana. There are some exceptions to this rule (for example, Bite or Boulder Roll), but they don't break the linear progression cardinally. QoT, however, has: (1) the same mana cost of 500 for both GS I and GS II, and (2) a sudden cooldown increase from 10 s for SW II to 15 s for SW III. These features (or glitches?) are unique and should be taken into account by any serious QoT player.
Levels 11-15: Your role changes radically. Start to demolish mid while your teammates begin to push portals. Be cautious and methodical, synchronize your ground spiking with creep waves. You'll both destroy mid towers and get to 15 very quickly this way. I don't think this strategy can be countered if you have decent teammates. If 2 opponents will try to gank you, they'll lose one of the portals, and 1v1 no one will stop you, given GS insane radius.
Levels 16-20: Depending on the route taken at 15 and game length, either get Bulwark and camp at enemy valor flag, or just camp at valor flag while Mulching. You can assist in taking side portals, but don't stay at flanks unless absolutely necessary, your potential is somewhat wasted there.
III. Game dynamics and support mentalité
"What are you doing, ffs!!! Trying to take on that giant 1v1?! You do no damage, dude!" (c) doggu
"Sed, why so low hp? Why so shit items? Are you noob?" (c) McLoover
"OMG!!! Do I have a noob Sed on my team?" (c) Iz_
"IN1, you should never pick QoT again!" (c) iamKira
"Maaaaan! I cannot hold two portals alone!" (c) renz0kuken
Be forewarned, you are likely to hear much worse things in noob/mid games, where the players aren't exactly renowned due to their skill and knowledge. All those guys I quote are undeniably pro, and have a firm grasp of general game mechanics. Still, people will get frustrated and irritated by your fragility and, ahem, 'uselessness' (the latter of which is an illusion, of course, but who cares?). Thus, the main virtue of any dedicated support is patience. However, there are some objective problems, rooted in game dynamics, any support-oriented playstyle will face. I intend to cover them to a degree in this section.
A. Direct/burst damage
Sedna: The ability to momentarily deal large amounts of damage is probably the most crucial aspect of DG combat mechanics. The only way a Sedna can actually contribute to the team in this respect is by leveling up Pounce at levels 1-10. Pounce is a strong nuke for a general: no debuffs, granted, but 1000 damage at level 10 is good. Postponing Pounce to end-game levels = decreasing its effect drastically due to gradual hp stacking by opponents. At level 10, 1000 damage is a freaking strong nuke; at 15, it is still passable but much less efficient; at 20, you will most likely tickle your opponents with it. Strong early Pounce is the way to mitigate otherwise weak Sedna levels 6-10, and, more importantly, to help your team in ganks. Playing late Pounce may be viable in 2v2, but 3v3 I won't ever risk it. The fact is, it will net you some kills.
Sedna's AoE problem can be remedied in a number of ways late game (see artifacts sub-section in Sedna section above), when it becomes truly relevant, but early game your only option is Nature's Reckoning. While an interesting item that helps you farm, sometimes works wonders in PvP and adds a random unpredictable damage factor (always nice vs skilled calculating opponents), I don't think it's essential, unless you are fighting a minion-heavy matchup -- that's where I can justify its taking a hp item slot, the dps increase will be too significant to ignore, glitch or no glitch. Otherwise, survival is a far more pressing matter for Sedna in 3v3, and getting NR in place - and for a price - of, say, Nimoth's is unwise.
QoT: QoT is the exact opposite. Probably the best AoE skill in game, but pathetic burst damage. A skilled QoT that doesn't get SW III (and the only way a skilled QoT will ever get it is by misclicking) can do a maximum of 1125 damage by GS IV > SW II. That's pathetic for a 2 skill combo. The situation changes late game, when QoT has a lot of minions and catas+giants are out. GS IV in a creep wave hurts. A lot. Overall, a QoT gets yelled at a lot less than a Sedna for being unable to deal significant damage: her skills are all long-range and very convenient, and early game that ranged Cleave attack in packed form makes a lot of difference, melting monks, creeps, Rook towers and DGs at once.
B. "Low hp, shit items"
Both Sedna and QoT will get Cur1, unless there is a Reg on the same team. Frequently, they will also get FS1 and Cur2. The total cost of those upgrades is 5600, which is roughly equivalent to a decent kit comprised of Vlemish + Banded + Unbreakables + Nimoth + Scalemail. So yes, you do gimp yourself to the extreme for the sake of greater common good, and skilled opponents will try to take advantage of that. Good teammates will buy you some basic armor items if they see you are focused non-stop (and you should be), but counting on it in PUG games is usually utopian. For some reason, most players (especially those belonging to the huge "high win %, mediocre skill player sticking to the UB, with a possible Shield III-whoring Oak alternate" caste) will rather let you die, then blame you for being a noob, than buy something for you. Some people will also get genuinely irritated if you won't have locks or sigils. They aren't evil, they just don't understand. What happens is a direct result of failure to comprehend basic game dynamics: they take those cit upgrades for granted, generally assuming you had a lot of money (and why not, if THEY had a lot of money, eh?), but you've probably spent it on something stupid, because it's obvious you are noob and don't just Spit > right-click or Shield > tp 24/7.
C. Teammate-dependent attitude
You depend on your main carry. A strong carry player + a good support will have a much better synergy than just two good damage-dealer DG players will. In 3v3, this principle is especially relevant. You will win a lot of matches if you find a good carry partner. Nothing to be ashamed of: you aren't a parasite, you are a symbiont, and smart players will understand it quickly, provided you do your job right. Personally, I have a list of recommended/not exactly recommended carry players (the recommended list includes some mids, not only the likes of Kira or ppapanek). The corollary of this principle is your playstyle should be more team-oriented than your average carry's. Communication is the key: pinging when needed, typing about intentions/things done/upgrades bought in chat, playing the game zoomed out are all essential. Also, don't rush in to kill, but rather assist (some people find this rule very hard to obey). It's not a competition, for God's sake! As Sedna, I usually steal a lot of kills because Pounce is such a great finisher: it's not a big deal, as long as you aren't egotistic and save the stolen kill money for creeps or J-Treads your UB needs.
