Rather than calling into question his trustworthiness or conviction in a narratively satisfying way like for instance just running away in the Astral Plane because he doesn't think we can win, you're left with "so he was full of shit all along." It likely ended up this way because it made sense for Daisy and they just didn't adjust it enough.
No? The Brain's orders reach the prism and are now almost relentless. He would have been quickly forced to obey anyway.
Wind mill Act3
Mind Flayer: I cannot risk this body. I have heard the call - we will all be needed for what is to come.
The way I see it, trust is irrelevant in this current configuration.
He is stuck with us for lack of a better option (if it's true that some tags linked to some dialogue lines have been unused/abandonned, none of them specifically concern his lines about "trust", it seems it's pure bluff), and because he is a control freak. It's probably better for him to take things into his own hands and choose his next action while he still can, rather than waiting for external factors like Tav's mood to decide his fate.
And even if Orpheus were to begrudgingly agree to cooperate with him, no one could pretend to ensure the Emperor's protection once his task is fulfilled.
There would be a lot more wrath involved, and the Emperor won�t take the chance and it won�t be possible to convince him as a non-illithid, but either subduing him instead of talking to him (Otto�s dance plus vines plus net arrows)
He should be cautious enough to keep reading the surface thoughts of his allies, especially in such a critical moment. Not to mention that he normally already forecast this option (You can make only one move at a time. But the Netherbrain calculates every possible move at once. It knows what you will do, it knows everything you could possibly do. You cannot outmanoeuvre it. To defeat it, you would have to think like an illithid. Better yet, be one.)
or tricking him into transforming me into an Illithid and then convincing him with the vast Charisma bonus
It might be even worse. The game should have pushed its intention and penalised both the ones who commune with the big tadpoles that the Emperor specially nurtured and the Tav who communed with him at the deepest level by preventing them all to disobey his decisions.
Emperor's hideout, the githyanki Ch'r'ai Har'rak: Most illithids prefer to dominate their thralls through transformation, yet here you stand, untransformed(/only partially transformed).
Imo that's the limit of the scenario, if you give to the Emperor the power he should have then, he would totally steals the adventure from the players.
And it could actually add another hard choice: if you have a companion, the Emperor could ask you: �If I take this risk, will you leave (name of companion) and join me after we defeat the brain?�
It is clear that that�s something he wishes for (�you are exquisite!�), so that�s additional leverage. And it preserves the plot aspect of forcing a hard choice just before the end.
I'm not sure why he would even ask? Thrall-bond is superior than love (The Emperor: Not love, no. What I had with Belynne Stelmane was much more than that). And if it's for dirty business, exceptionally strong personality or not, Tav's mind and soul are altered beyond recognition, romance stuff will be finished soon enough for them to ultimately knock the Emperor's door and seek for his guidance.
"You are... exquisite." is probably just the best move to do since Tav is very vulnerable as this moment, they need both reassurance on their appearance and encouragement, not only to keep them at their best of efficiency but also to convince the companions to imitate them.
It's too late to complain since this possibility is already in game anyway, but I’m not fan of the idea of being able to convince everyone to commit the things they disapprove the most with just a dice roll.
