Energy drain just does less damage than fester. Fester is 300 potency, energy drain is 150.
Now, if you were running out of mp, it is a -marginal- increase (well, potentially more than marginal) to use energy drain instead.
Each energy drain is ~240 or so mp.
At worst, it will let you pump out ~3-4 ruins, which is more potency than the difference between fester and energy drain.
Since you should have a bard in your group (again, so much utility for casters, and the fact they ignored thunder when they mathed out summoner mana use), you shouldn't need to use energy drain.
Do NOT use energy drain if you are not running out of mana.
Back to talking about cleric stance:
Anyway selene's aura seems to reduce my cast times by around maybe .3 seconds? I think? I can't really remember.
Lets round that up to selene's bonus is 0.5 second cast time decrease (its lower than that, iirc).
In 3 minutes, you spend ~40 GCDs on DoT upkeep. With selene, thats 80 seconds spent, as opposed to ~100 seconds spent. However, its only up half the time, so lets split the difference and say half the DoTs are hasted, half not. So thats 90 seconds spent.
You now have 90 seconds to ruin. "normal" ruin would take you 36 ruins, but since we're half hasted, its 40.5, over the ~32 ruins baseline (w/o any hasting).
So you gain 8.5 ruins. However, you lose ~6 GCDs due to losing Garuda's contagion. So you only gain 2.5 ruins.
That's a total of about 200 potency, over 3 minutes.
Garuda pumps out close to 5544 potency over 3 minutes, not counting contagion.
Now, lets assume that you have exactly the same stats (you'll have lower) and that cleric stance is a pure 10% dps increase, since based on your thing its basically that if we take into consideration fester vs aero.
First of all you lose RS. RS is ~3% damage contribution. So your bonus is now 7%.
Summoners do a personal potency total of ~16709 over 3 minutes. Your 7% potency bonus is now ~1170 potency.
So your total potency bonus is 1370. Garuda is 5544.
You lose a personal potency of ~4174 over 3 minutes, or 23pps or close to 20% dps loss.
Lets assume, that we have perfect caster dps, pure hardcasting, so receives full effect of selene's buff whenever its up.
Lets also assume this caster does the same total potency as our baseline summoner, because that way everyone does the same damage.
This caster does 22253 potency over 3 minutes, aka over 72 GCDs, and ~309 potency per GCD
With haste up half the time, he instead has 81 GCDs. He now gains ~2781 potency.
So in this perfect scenario, scholar would be more group dps if you have 3 casters in your team, and the other casters are all BLMs.
I want to have you realize how unrealistic this "upper bound" is, and even if so you would have to have close to an all BLM party for it to be beneficial.
If they were all summoners you would have to have about 6 other Summoners in your party to break even, obviously not going to happen.
It is "viable" I suppose, but pretty awkward in practice I would assume.