I don't understand why omeluum isn't a valid option instead of the emperor if you meet the criteria that you haven't killed him, sold the ring he gave you, took the ring from him, and if you saved him in the iron throne and meet up at the society.
From a story writer's perspective, it goes like this: you want something to happen, and then you make up reasons for it and event chains that actually make it happen. So the *primary* answer to "why must someone turn into a mind flayer" is that the story writer(s) at Larian wanted things to go that way, probably in order to make some point. Trust me, something this important almost never happens in a story unless it is explicitly desired by the writer. And the reason why Omeluum doesn't come in is because Omeluum would have made that what they wanted to happen less plausible.
So in order to convince someone at Larian to change this retroactively, should that be a plausible possibility (which I believe it is not, but I still hope they'll do it), pointing out ways this could've worked out differently will do absolutely nothing. What you need to do first, is convince whoever might be able to change things, that this f******* idea that someone has to turn into a mind flayer was bad to start with. So bad, in fact, that it merits retroactive change.
That is why I did not point out alternatives in the OP, but concentrated on why it is such a disgustingly, so heartbreakingly bad, self-contradictory and broken way to end the story. And if change should be coming at some point, no one who wanted it will care a great deal about alternative ways to bring about the new scenario either, as long as it isn't nonsensical beyond our rationalization power.
