The Emperor wasn't the one who imprisoned Orpheus (that was Vlaakith 1) and a close look at the timeline reveals that really the Emperor has only been manipulating Orpheus for a short while to aid Tav and the companions - in game terms most likely less than a couple of weeks. He didn't even know the prism or Orpheus existed until Gortash sent him off to find it and that's just before the game starts.
Given that I feel Orpheus, who seems to have a pragmatic streak, and in the game as currently shown knows perfectly well that an Illthid is needed could be reasoned with.
Maybe just release the mind control on him so that the Emperor could reassert control quickly and easily and make him feel less vulnerable would be a way to go?
After all Orpheus isn't free until the hammer kills the chains. So the Emperor really does - if we are prepared to go along with it - have the upper hand.
We release the mind control, talk to Orpheus, he apparently understands the situation very quickly in the game as stands so it should be no slower for him to grasp the necessity for having the Emperor along. He can be as nasty about it as he feels necessary but if he truly sees the threat and that the Emperor is part of the solution then he should be able to be convinced (V High Persuasion check) maybe have to have Lae'zel in the party so he can see through her eyes what is going on.
If we pass the check we can release him and he and the Emperor are short term, somewhat uneasy allies, but capable of working together.
If we don't then the game continues with what is currently in place.
