Omeluum would be great as an option, other players in the thread have already written about it, I can only join in and agree.
However, in your version of the game - yes, I agree, your character is really trapped in a choice without a single good option. Each option is tragic and the choice is really hard to make.
I myself can say I was lucky enough to take Karlach with me in the first playthrough, although I was very disappointed and disenchanted with the character, she was good in combat and I took her with me on bosses. I really wanted to save Orpheus and ensure Lae'zel had a bright future. But, if she's sitting in the camp, then yeah, in a game with no spoilers you wouldn't guess she could do that. I think Larian had a reason to let exactly her have that ending - Karlach wants to stay on Faerûn very badly, at any cost, she's willing to die just to avoid going back to Averno, and will only live if someone is willing to go there with her. Her quest with the engine didn't get developed further for some reason, and without the option of becoming illithid with those Tavs who have their own personal lives and don't want to go Averno with her, she would only survive at all if Wyll becomes the Blade of Averno. After becoming a Blade of Frontiers, Wyll has no desire to help her. And this way she has another option for survival.
Thiking about it a little futher, in perfect world they would all react differently ...
Imagine that if you could as every companion ... but those who would be against it would get super high persuation/intimidation/deception check ... like 30, or even 40, but not 99 (xD) ... something only super-focused character could pass, still something a player could achieve.
But then, when you would fail ... and lets be honest, most people WOULD fail that check ... they would become hostile.
But then again, if we would knock them out, we would be able to turn them against their will.
I mean, for hells sake, we can be evil enough so we quite literally "drown the whole world in blood of our victims" ... lets not pretend that some of our characters would shy away from such option.
Yeah. Exactly that only “drown the whole world in blood of our victims”, but interesting evil roleplay was not brought in. So to have not “isolation” and a bunch of corpses along the way, but a separate full-fledged storyline, as it was in BG2. Even the evil DU ending was made so that I now have to give any of my evil characters some sort of motivation to reject Bhaal, just so I never end up with that. But, in general, you're right, maybe my evil character wouldn't mind playing such an “evil joke” with Wyll, manipulating the stupid ducal son with the ideas of “self-sacrifice” and “saving the world” to then laugh with Astarion at his tentacles, lamenting that his horns have disappeared (horns and tentacles together would be even more amusing).
Or with Minsc - that might be even funnier! Yes, I'm already changing my previous point of view, having imagined a Minsc-illithid, I would definitely do it with him, terribly curious how a Minsc-illithid would behave...
Well, if we think about each character in relation to the difficulty of the check and the degree of their reluctance...
Lae'zel - will agree for the sake of Orpheus and for the sake of her people. The persuasion must be based on that.
Wуll - yes, you can persuade/deceive him, he's pretty stupid (Mizora will confirm this) and manipulatable, and generally a dependent character (the only one in the group who can't make his own decision in his own quest). You can use his “heroism”, I wouldn't make the persuasion check very difficult. Intimidation can be harder to deal with, deception and manipulation always work better with “stupid good” alignment.
Minthara can be persuaded, but not in a “form of redemption”. First of all, she had nothing to “redeem”, she was under the control of the Absolute and her life was already hard. Secondly, Minthara is intelligent, and she would never fall for such manipulation. She can be persuaded by the fact that by becoming an Illithid, she will gain immortality and power. In the finale, Minthara will make a grandmaster's gambit - she will subjugate Brain herself and become the Absolute, and it will be a separate “evil” finale with Minthara in the lead role and the collapse of all Tav/Du's hopes and plans. Although with the “redemption” persuasion could also work, but there should be a high passive Wisdom check - will Tav realize what Minthara is really up to? Same Wisdom check with Intimidation.
Astarion - yes, I realize that it is possible to pressure him into doing it after all. That won't work with Ascended Astarion though, Astarion is now able to stand up for himself. He may try to convince the player to choose another victim, arguing that with his new powers, he will be a more useful ally in the coming battle. If the player insists, no amount of persuasion/intimidation/deception will work in this case, Astarion has absolutely no intention of doing so. It would be illogical and unprofitable for him to fight the player, considering the strength of the whole group and the danger of the Elder Brain, he might turn into mist and leave, and won't participate in the battle (and won't come to the party in the epilogue, he might write a letter). Unascended Astarion may be forced and surrender eventually. Persuasion and deception is unlikely, intimidation just might work, Astarion must realize he has no other way to preserve his own life. Astarion will hate Tav very much after this. After becoming an Illithid and gaining power, Astarion will do the same thing as Minthara, become an Absolute himself and enslave all the other companions, including Tav. This kind of ending with Astarion and Minthara would happen when the player wanted a “good” ending (and it would be a gorgeous sudden plot twist), when the player themselves wants to take over the Brain - there would be a battle and confrontation similar to what happens in this case with the Emperor.
Gale is the big question with his sphere. I don't think it's even possible because of the sphere. And Gale has the option of self-sacrifice in the finale when he blows up the Brain and dies himself. You could consider that an alternative. Have you tried in a game as a Githyanki to persuade Gale to sacrifice himself and blow himself up in the finale? That might have solved your choice problem.
Minsc - persuasion is practically guaranteed. Shouldn't be particularly difficult, Minsc is considerably dumber than Wyll. Persuasion shouldn't be difficult, Deception will be easy, but Intimidation is probably impossible. In the case of an Intimidation attempt, Minsc will attack you, most likely.
Shadowheart - I don't know what string you can pull to manipulate her into agreeing to this. Maybe only Intimidation (and that's not the fact that she wouldn't prefer death). Shedowheart is a religious character, she either finds her faith in Selune or is a devout servant of Shar, and illithids are repugnant to the gods by their very existence. Those who become illithids betray their god forever, and lose any chance for postmortem in the deity's domain (the souls of illithids in DnD lore are sent to the Far Realm, the gods have no power over them, which is probably why Withers says that “illithids have no souls”, for the gods these souls don't exist at all). The character serving the deity, the cleric, is unlikely to ever agree to this blasphemy, more likely to die and go to the domain of their god.
