We actually know quite a bit about Orpheus. We know he is a githyanki, which means he is 1) fiercely loyal to the cause and 2) really, really doesn't like illithids. These two things suggest that Orpheus can be a potential, and a rather useful ally.
And if we choose to not immediately dump a potential ally, we are betraying the Emperor? Even if he is right and Orpheus will attack as once freed, we can just kill him and let the Emperor eat hsi brain. There's literally no reason to not free him, other than the Emperor being a bastard, which he was throughout the entire game.
I'm sorry, but just knowing his race does not count as knowing quite a bit. (Also, the githyanki cause is only fair to tolerate if you're a githyanki yourself. Otherwise you are his potential enemy and slave. But I suppose you mean the enemy of my enemy etc, ignoring that the Netherbrain IS the Emperor's enemy too.)
And I've already said previously, the reason we can't free him with Emperor on our side is because
Larian wanted to force us to choose. So you're right about one thing: there's no reason to not free Orpheus in any reasonable universe. Give us persuasion checks, or whatever, but we need an option to conclude this in a reasonable manner.
Those are our only options and the The Emperor lied to us and manipulated us. Load the appropriate saves or search on youtube - the Guardian gives us advice on where to find a cure while encouraging us to make the infection worse. As the book tells us judge a mind flayer by their deeds, not their words. The Emperor tells us they never lied to us but that's false - they were leading us down a path towards transforming into an illithid while dangling the promise of a cure in front our eyes.
And the emperor has clearly lost itself - it killed its best friend, destroyed the mind of a duke of baldur's gate and transformed from the roguish hero of the city into the leader of a front organization for a devil. There's a good deal of support for seeing the Emperor as an evil, soulless monster but, for some reason, that's not reflected in the dialogues you have with it.
Yes, the Emperor enabled us to find the source of the problem. He knew that we were up against an Elder Brain and knew that having more power would aid the cause to defeat it. He absolutely was not lying when he said he was on our side, and that he wanted us and himself to be free. He never even asked you to become an illithid (you could give the stones to him), and before that he merely encouraged you to use the powers (and the Astral tadpole).
I don't know what you were reading into the dialogue but I never felt lied to. Did he present his arguments in an appealing way? Yes. Could he be more transparent? Certainly, but that risks you refusing to work with him, and becoming enslaved to the Brain. That's not a great strategy.
Also, he killed Ansur in self-defence. That's very clear, and supported by Ansur's dialogue that Ansur was going to kill him.
