Since I just had that scene again: Yes, either you should be able to persuade those two or ... bring Omeluum to the fold. I mean, we know, that the Emperor is still somehow controleld by the Elder Brain, but Omeluum isn't, so if you save him in the Underwater Prison, make him available as your illithid helper. It makes sense - Omeluum was willing to give his life to save the Duke, the Emperor wants to survive at all costs - he even killed his best friend (and lover, I guess?) for it.
Another thing is, that LAe'zel should at least offer to become illithid in Orpheus stead, since she basically worships him. I don't see, why I have to change, but she sees him as important, so she should at least offer.
