How Priests/Monks Work (VITAL for a Sedna):
Every 5 seconds, monks do a for-unit-in-range check.
First, it will check for allied demigods. If it finds a damaged demigod, it will heal 10/12/13/15% of their max hp. If they are buffed by Healing Wind II, then they heal 15/17/19/22% of their max hp. It will then apply a debuff to the healed demigod that lasts 8 seconds and prevents them from receiving heals from MONKS OF THE SAME LEVEL. This means that both a Cleric and a Monk can heal the same demigod twice. But 2 Monks cannot.
For this reason, if you are a general and there is a Sedna on your team, you should not purchase Monks as your first purchase. Instead, buy Banded + Scaled Helm, go farm on the mana side and have Sedna split her monks. Once you have 1500 gold, go purchase Clerics and give HER a Cleric.
High Priests only heal 1% more than Clerics (an 8.3% improvement, ignoring their extra HP, armor and damage). For Sedna, they heal 2% more than Clerics (an 11.8% improvement). Bishops heal 2% more than High Priests (a 15.4% improvement). For Sedna, they heal 3% more than High Priests (a 15.8% improvement).
To maximize your % improvement, if there are 2 generals on your team, the Sedna should ALWAYS get the High Priests, and the other general should get Bishops. Never ever ever the other way around. (11.8 + 15.4) > (8.3 + 15.8). (12.9% more efficient).
The exception to this is if there is an enemy Sedna. In this case, stick with Monks/Clerics instead of High Priests/Bishops.
If a Priest doesn't find a damaged demigod in range, it will then check itself to see if it is damaged. If it is and will not be "overhealed" (eg, if they only lost 5 hp, and the Monk heals for 200, there isn't much point for it to be healed since it will "overheal" by 195 hp). Priests heal for 200/230/260/290. If it won't overheal, it will heal itself.
Finally, it will go through all the regular units (including other monks), and check to see if they are damage and, if they are healed, will be "overhealed. This means that higher-level'd priests will less often "waste" their heals on grunts because it is easier for them to overheal.
What this means:
In a 1v1 situation, 2 monks will usually be able to "optimally" heal the Demigod every 8 seconds, even with AoE abilities (because one monk will heal the other, which will then heal the demigod once the debuff expires). However, if there is only 1 monk, it is very susceptible to AoE. Any decent AoE (over 200) will reduce the frequency of the heals by 25% (from every 8 seconds to 10 seconds. It will heal the Demigod, apply the 8 second debuff. 5 seconds later it will heal itself. Then it has to wait 5 seconds to heal you).
This is one of the reasons why it is worth it to "tag" the enemy monks with your monks, and one of the reasons why NR is so useful for Sedna, even one with Counter Healing.