One of Larian's older development updates they talk about how they account for player choice in regards to things that MUST happen for the purposes of not breaking the story and they talked about Shadowheart's artifact and how no matter what the player does that artifact always has to end up in the hands of the player.
And in regards to Omeluum no, there are mind-flayers that manage to break free of the influence of their elder brains but that is not the same thing as retaining the memories and behavior of the host. The host that the tadpole transformed is gone.
As for whether ceremorphosis can complete and so on we don't know but in the most recent stream they did say we can go far, really far, down the path of accepting power from the tadpole. What that actually means and precisely how far that is exactly we can only speculate.
