As for the "strong mind", it's not the resistance to illithid influence, but retaining the memories and the personality upon transformation is what's an issue. An illithid kills its host upon completing ceremorphosis. Orpheus simply changing into one with no explanation given (one would assume he actually becomes a mind flayer rather than simply assumes a form of one if it's irreversible and it specifcally requires an actual mind flayer to counter the brain, for whatever reason (it amounts to focusing on a spell, which you'd imagine a powerful enough spellcaster with an intelligence/wisdom to match would have been able to do regardless)) is another moment that is hardly making much sense.
Orpheus used a todpole. In D&D soul/mind can be separate from body. So it is possible to have body go through physical change but keeping the soul/mind intact. It is a magical world
But nonetheless, that physical change does affect his mental status. That's why he sought death after the final battle.
