Say, Crimsom, did you just write a justification for why Ascended Astarion is a good guy actually? Because everything you wrote in the (edit: by now second to) last longer text is applicable to our dear scheming vampire lord. ^^
He is afraid and wants everyone to be at their very best when the final showdown begins, yet despite everything he not once forces it upon us despite knowing we're putting everyone at risk. Which is why if the player refuses to become an illithid, he'll take it upon himself to consume Orpheus and proceed to the final showdown because that's who he is. Unlike us he'll do what needs to be done to survive.
No, the ones who do what needs to be done are Gale (unless you talk him out of it) and Orpheus. The Emperor does not face any repercussions from handling the stones, it already is a mind flayer and eating Orpheus is just a delicious snack and a power boost for it. Everything it does is convenient for it. Gale by contrast is also terrified of the Brain winning, he is so terrified that he accepts morally corrupt solutions to have better odds for the final battle, and there is an possibility for you needing to go through three rounds of talking him out of exploding because he believes him dying is the safest solution to the problem. (Which, true, it's easy mode.) It was the most uncomfortable conversation I ever had with him as my partner. Three rounds, three, on top of the ones we had in the Astral Prism. And Orpheus is much the same. though with less fear, more ratio and a lot less words.
As to forcing our hand, after accepting the first additional tadpole, our hand is forced. If your player character fails the WIS save, they are forced to consume the Astral tadpole and become partial illithid. Trust the Emperor once and you are done for.
Fear of not being enough (low self esteem) is also the motivation for both Astarion and Gale to follow down their ascension paths. Both think they are pathetic as a spawn or as a human and try to be something better, not seeing that they are pretty awesome the way they are. Giving in to those worries changes them both for the worse. To paraphrase Lae'zel, trusting in your own immense talents and the mighty company you keep, should be enough - Power is only a good thing as long as you do not loose what makes you you for it. Which is why squid Orpheus asks for death in the end. He is most scared of loosing himself.
