I agree with your main point that WHM and SCH will work well together in endgame, both in 8man and 24man.

However, I must correct one of the things you said... WHM do have a way to get mana back: Shroud of Saints. It can be used strategically to lower hate, but it also has a Refresh effect. It helps WHM regen mana when there is no other sort of Refresh. SCH, on the other hand, have to really plan out when they're going to use their Aethercharges. It makes the most sense to use Aetherflow on cooldown so that you get the maximum amount of mana back, but you have to plan ahead just in case you know you'll need to use Sacred Soil or Lustrate soon.

I believe there are two main reasons people tend to stack WHM/WHM rather than WHM/SCH:
1) Not many people know how to play SCH well. It is a preemtive healer, for the most part. You shield when you know something bad is coming. You save your charges for when you know something is about to happen. Scholars must KNOW the fight. (This is appropriate considering the entire Arcanist storyline is about math and calculations.)
2) WHM has been out for much longer than Scholar, and is more of a REACTIVE healer. A good white mage will know the fights as well, but WHM can be more reactive with their heals. If people mess up, WHM can toss out large cures fast without penalty (besides mana). If a scholar has to throw out an unnecessary lustrate, that's healing that could have went toward the tank.

Scholars cannot 'make up for' the mistakes of others as easily as WHM, but when EVERYONE knows what they are doing, Scholar makes the fight easier for EVERYONE. (Those shields are amazing). Scholars are also known to solo heal Titan, so they CAN pull off everything that is necessary...