Possibly someone's done this already or it messes up some other bit of SoD (I still haven't played it!) but a simple minimal fix would be to replace Semaj, the mage in the final fight with Sarevok in BG (who has no backstory at all as far as I recall and is probably a self-insert of some developer called James), with Korlasz, give her a min-hp ring, and have her dimension door away when reduced to 1 hp. (It would require a bit of care to synchonize with SCS's final Sarevok fight but that could be done.)