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.
But I just realised something. With all the talk about the interview, I also thought about which is my favourite scene with Wyll. I'd say, it's when you recruit Karlach first and he then visits your camp to hunt her down. The scene has some really nice drama and tension to it .... but it is also not really about him either.
