This is something I've been trying to wrap my head around... when the Body Thief ability is used, the host is not dead until the Intellect Devourer leaves? It says on page 191 in the MM that the creature is ejected if the brain is restored by means of a spell like wish. Would regenerate work? I don't see a reason why it wouldn't, but I want to make sure I am not misunderstanding considering how difficult it is to otherwise remove a devourer.