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.
