There's a difference. Becoming a Mindflayer means loosing your soul. Gale has a means to make sure nobody has to loose their soul and the crown is destroyed for good by using the orb, why should he offer the far worse solution? In Karlach's case it's also not a cure, it's her desperation. She clings to her life so badly at this point that she will do anything to remain in Faerun even if it isn't really her any longer - though she might not really understand the issue, her Int isn't really up there. In Wyll's case it would make sense, especially if he has already forsaken his soul to Mizora. That would be an actually good twist to get out of a devil's contract.
Unless Larian rewrote this part of the story in patch 5 what you posted about Karlach makes no sense. The way it played out in my game, Karlach doesn't know (or anyone in the party at this point, that is player's metagaming knowledge) that the mindflayer transformation will "fix" the infernal engine. She offers, because she is dying and feels that way no one else needs to sacrifice themselves. That is the opposite of clinging to life at any cost.
That anyone transforming loses their soul is a problem in itself, because you can learn in the game (unless Larian rewrote that part of the story) that the soul can be restored through divine intervention - but only if you play as Gale. Mystra will offer if you choose as Gale to turn into mindflayer. Withers however will not mention it, and there is no other way of the player learning that, so that they could e.g. use the high level cleric ability.
