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.
The choice is literally to work together or let the Netherbrain complete the Grand Design.
I can't possibly see a world where working together is the greater evil compared to letting the Netherbrain win. Even where you free Orpheus, you can see when he is released that he is resentful, but that the greater good must take over and that he must take the pragmatic route.
