I think I mentioned on a different spot of the thread, but I actually suspect that using a blade of light on an Ascian might have the effect of sundering that individual's soul. Not destroying, but splitting in the same way that Hydaelyn split souls originally--possibly just in half though rather than fourteen pieces. Split souls have more limited abilities and amnesia on past lives as Amaurotines. Whether that is a better or worse fate in the eyes of Ascians remains up in the air, but as a player who thinks there is value in certain limitations I actually think it would be a mercy. Probably the best fate they could ask for there, but one they would never willingly take. If they weren't actively trying to genocide people would make for a weird ethical question, but since they are imo it would address a lot of issues very neatly imo.
I could see people disagreeing with me on this on maaaaaaany fronts, but imo and as the path I'd personally envision for WoL I'd run with that quote "To ignore the plight of those one might conceivably save is not wisdom--it is indolence."
Since Zodiark is the will of the star rather than the star itself, I think he might need to be destroyed while in pieces to free the souls trapped within. Souls within, coming from split fragments of Zodiark, would in turn be fragmented--which in turn would potentially protect them from the Terminus event and allow them to reincarnate. They wouldn't be their original selves, but they wouldn't be trapped in limbo anymore and would have the opportunity to experience life again. Since seven rejoinings have happened and the void is a wreck, that would probably amount to the source having a big boss fragment, the void having a big boss fragment (by virtue of excess darkness and stupid environment), and all the rest being standard sundered-Zodiark size.
The reason the Ascians kept running into resistance was near as I can tell that they were insisting on complete souls to resurrect those sacrificed as they were originally, which could only be managed by rejoining Zodiark. Rejoining Zodiark requires genocide. Frankly idek if the souls would have been the same after that misadventure anyway.
All that said though, if the principle they have in mind for getting souls back from Zodiark is sound and if the principle I described is sound, Lahabrea really might be out there somewhere in a half-digested state. Hard to say lol.