First, some general game changes. Full shield stacking is allowed: two scholars, a scholar+astro, or even two astros in nocturnal stance (why?) will now be allowed to stack shields. At the same time, shield overwriting from an individual healer will only happen if the overwriting shield has more capacity left in it (that is, it's bigger) or has 2x or more duration it will last compared to the current shield (those two combined should keep things sane). This means double scholar comps are no more gimped than double astro or double whm comps and that nocturnal astro + scholar is no more gimped than diurnal astro + whm which I think is reasonable and
So, second off, Scholars lose deployment tactics. This is necessary to make it so the only way to try to cheese a mechanic is by both healers manually casting their strongest shields on everyone which is hard to do with time constraints and generally not something I think we'll need to worry about, especially if encounters are well designed.
The crux of the rework then becomes the following:
Remove dissipation and cleric stance and replace them with 'fairy' and 'solo' stance.
In fairy stance the fairy is out and can be used like normal while every CAST of a dps spell reduces the cd of the fairies abilities by an amount equal to the cast time of the spell (to encourage either regenning via eos or actually giving a reason to actually use Selene).
In solo stance the fairy is unsummoned (ala dissipation), but ALL damage (including dots so even if you're actively healing in this stance you're still getting the benefit from DoT damage) provides a stacking shield (separate from the regular shield) to a random party member equal to the damage dealt. I would tentatively say that Quickened Aetherflow needs to be buffed when in this stance or else it needs to drop the recast time of aetherflow by 10-15 seconds automatically because this stance will not have passive fairy healing (since the shields going out are random they'll be mostly usedful for aoe damage and not for tanks) and thus will either need more energy drains to sustain mana for direct healing or else more casts of sacred soil, lustrate, and indom to replace the lost fairy casts or perhaps most likely a mix of the two (with aetherflow potentially being on a 30s cd with quickened aetherflow in this stance). Heck, since the fairy gauge is being sacrificed they could even make it so you don't get a direct cd reduction in aetherflow but make it so that in this stance aetherflow generates 4 stacks so it's in a way like a ~20s reduction (~15s for a stack and then another 5s from using the stack).
Basically, one ends up with the choice between semi-passive fairy healing plus fairy gauge or 'dumb' dps healing (well, shielding) plus increased aether spell casts. At the same time, the issues with stacking shield healers is fixed through a somewhat cludgy removal of the obvious balancing issue in exchange for a slight buff to the fairy scholar stance (and the power of the solo scholar stance is thus slightly higher than I would suggest otherwise as well).