If SCH got Energy Drain back, why not do the same for WHMs?

[Archwizard]

Sch is different from whm because we want to use aetherflow on cd to gain mp, but we also don't want to waste our stacks because they build fairy gauge.

Sure. And on WHM if you're holding 3 lilies, you're neither generating more nor nurturing the Blood Lily.
There's nothing stopping SCH from popping Aetherflow for MP when they still have stacks, except for the fact that it's a waste, much like holding three Lilies or leaving the Blood Lily too long.

I think giving back stoneskin (untraited) to white mage at the cost of 1 lily per use would be a fair compromise.

While a weaker Regen or shield option consuming a Lily would be easier to "dump" on a tank.

If the argument is that the oGCD skills we have are more than sufficient for keeping a tank alive such that we aren't burning Lilies fast enough, how would giving us more of the same tools remedy that issue?
It seems like you're taking the "dump" part too literally here. The goal of dumping at all is to continue to gain some degree of value from them.

I like the idea, however to not make it bonkers as all heck, using a lily that way should not build up the blood lily. Otherwise it would be massively overpowered.

I don't think it would be "massively" overpowered, since even if you're nurturing the Blood Lily in the process, you still have to deal with the fact those same GCDs could be spent on, say, Glare (or even a rank of Stone, depending on the level in which the dump spell is introduced).
But having it not build the Blood Lily is still something worth considering, even if seemingly counterproductive.

IMHO I think the potency of the lily skills would need to be toned down quite drastically to remain balanced as an oGCD, it'd likely have us clipping horridly again and it'd just lose the current satisfying feel of the whole mechanic.

I'm not sure where in my post I gave the impression I wanted the lily heals to be oGCD. I love them in their GCD states since they allow for weaving and on-the-move healing, and the potency they offer gives me a tool for snap-swapping to a tank when casting a Cure II would be too slow.
I only mentioned that they're on the GCD for the sake of illustrating that it's harder to dump them than it would be to dump the oGCD heals Scholars have.