Orpheus would never work willingly with an Illithid as he believes it will betray him. He only turns into an Illithid to finish the fight and then will suicide as he is not sure even he can control it for muchg longer. Also consider that Emperor has essentially enslaved Orpheus for this whole time, not something Orpheus is likely to forgive.
Not strictly true. Vlaakith 1 imprisoned Orpheus and subsequent Vlaakiths kept him boxed.
At the point at which the Emperor works with Duke Tremayne in Baldurs Gate he doesn't even know the prism exists.
That came later, after he was sent back to the elder brain by an adult Gortash, then sent to find the artefact by the brain, found it (read: Stole it from the Githyanki) and got his free will back.
This is all very recent stuff and its all in the game with flashbacks.
So really the Emperor uses Orpheus's power and stops him from being freed but that was for a very short space of time, probably only a handful of months at the most. (I suspect its only a week or so as the honourguard were perfectly capable of downing the Emperor the one time we see them in action and I doubt he could have been fending them off solo for very long).
Pretty sure Orpheus could be pragmatic if approached right, especially if it saves him having to become Illythid himself - after all he picks up on the necessity for there being an Illythid in the attack squad very quickly. He doesn't have to trust him, it's an 'enemy of my enemy is my friend' situation.
