In 2v2 Sedna builds, dropping pounce for an early Inner Grace is the reason you're seeing better performance. Pounce is nearly useless in a Sedna mirror (in 2v2... 3v3 there are two other demigods you'll need to pounce), and being 5% faster enables you to engage and disengage at will... something the other, slower, Sedna has nothing to do to stop you short of getting Wyrmskin gloves instead of NR. Basically, Inner Grace is underpowered in all matchups except against Sedna. 

It's also always been my opinion that for a lower-level Sedna player, you should invest 1 point into Inner Grace because it gives you more wiggle room, especially early game. However, the "sigil and run away" won't work against good, aggressive players. 5% speed isn't enough to outrun a gank, especially when the gank includes any of Oak, UB, Erebus or DA.

Though against a DA-orientated gank teams I do tend to go Heal > HW > IG > SAVE > HW + IG > Heal > Heal > CH > Silence > Heal because it enables you to outrun him most of the time, even if he is clever and Warp Strikes to a monk. And Silence won't save you against those teams because of Chain Stuns usually. 

The vast majority of my end-career 2v2 games were with Thundercles, who does a hybrid BotS TB (usually). We both somewhat tailor our builds to capitalize on the Sed/TB synergy.

Although, yes, your damage from pulling 2 points from Silence into pounce, your mana consumption vastly shift. 

5 seconds of Silence + 400 Pounce: 1400 mana

3 seconds of Silence + 1000 Pounce: 1575 mana

You only get a 33% longer duration, then a 25% increase of timeby leveling up Silence II and III, so you do get a little bit of diminishing returns. However, the mana efficiency of Silence by leveling it up is amazing. 800 -> 900 -> 1000 mana. So you're paying only 12.5% more for 33% longer and then 11% more for 25% longer. 

Compare to Pounce II, III, IV. You pay for 31% more for 50% more damage, then you pay 24% more for 33% more damage, then you pay 19% more for 25% more damage. Yes, Pounce is getting gradually more mana-efficient as you level it but not at the same rate of Silence.

Put another way, Pounce I is 1:1 in terms of damage:mana. Pounce IV is 1.29:1 (29% more efficient, for 3 skill points, all of which compete with other skills for priority)

Silence is .00375:1 in terms of seconds:mana. Silence III is .005:1 (33% more efficient, for 2 skill points, only which level 2 competes with other skills for priority)

Finally, note that it is actually TWO pounces, due to Magnificent Presence cooldown drop. So it's really 800 damage + 5 seconds for 1800 mana. Or, I can pounce at the start and hold my second pounce so I have an interrupt for a lock or a port. 

With only 3 seconds of Silence and no MP, if you don't kill them they can just port out if you pounce. Finally, at level 10 and on it's VERY hard to fall below the 400 mana needed for a level one Pounce. So you are the "interrupt bitch" of the team. Leveling up Pounce can put you into a position where you don't have the mana for it. 

I'm not judging your assessment and before I left I started doing a very similar build (Heal > HW > Pounce > Heal > (5) HW II > Silence > Heal III > CH > Pounce > (10) Silence > Pounce > Pounce > MP > (15) Silence > MP > MP > Heal IV > IG > IG > IG).

I'm merely bringing some math to the table to challenge a few of your statements. 

Yes, those things always work. For example, I've used a modified version of your pure Ooze build while laning vs Spit/Ooze hybrids with great success. I'm quite sure any T1 hybrid UB would tear me to pieces in no time, but with mids-mids+ it worked and was quite entertaining. The 2 differences are: (1) I use Symbol of Purity instead of Blood: well, yeah, -550 hp early on, but neutralizing hybrid's biggest damage source is a greater benefit (not to menion the frustration it causes); (2) I take Spit 1-4 instead of Bestial Wrath 1-4 late game. Now, the trick is not to spit until the kill opportunity presents itself: no one expects pure Ooze to spit all of a sudden, so enemy UB will frequently fight until he's reduced to very low hp, as he feels he's safe if he gets out of melee range using J-Treads -- then it's Spit > late game kill, which can be super-crucial.

Symbol of Purity  

I use it on Oak (against a UB) and UB in all-assassin games. And in 2v2 Sedna mirrors ... 

I don't like sharing lanes with a Rook, I think it's a waste of a demigod choice (Queen, Erebus and Oak are so much better with Rook). 

That said, I have played around with Cape of Plentiful Mana. I do really like it and I think it's a really good way to boost your Rook, especially if they go Staff of Renewal. Work in an early point of MP and don't get Counter-healing at level 8 (since you're hiding in his towers it's mostly useless). 

You have to be extremely careful, however, because you're going to be very low HP. For this reason, Cloak of Night can be better. If gives you a nice large mana pool to keep your Rook alive (and most teams will try to kill slow-old-Rook a lot), and you can Heal -> Blink without putting yourself at too much risk, whereas if you go Cape you're much more likely to be focused and melted. 

Basically, with the team (a Fire TB in particular) a Pounce CoN build is probably going to be the best. Fire Aura makes Pounce helluva lot better. Your team will lack interrupts, especially since you won't be getting Silence early on, but you'll have a lot of damage. You may want to prioritize heal more than Pounce (so Heal > HW > Pounce > Heal > HW II > Pounce > Heal > Pounce > Silence > Pounce), espiecially if you're fighting against big debuff demigods).

For items, get Plenor instead of NR. NR is useless with/against Rook and with a heavy pounce and heal you'll need the larger pool.