Emperor still has his own mind intact
This statement is debatable ...
Not really.
When a Mind Flayer consumes a brain (Including the initial host) they acquire a shard of that person. A small echo of them, which intergrates into their Illithid self in the form of an incomplete memory, a personality trait, a fear etc.
They taste the full person as they consume the brain, but they only integrate a small portion of them into their being, which simply merges with the rest of their self (Quite unlike the Ship of Theseus, which is all about replacing parts) - Unless they notice it and purge that shard.
Yet the Emperor has complete recollection of his existence pre-Illithid. Not just "Oh, I vaguely recall being this person" but all the details of his life. This mind also takes full control completely once he goes into proximity to the artifact, completely suppressing the entire Illithid mind, indicating that it is completely separate to the Illithid mind and hence, not merged into it like a normal Illithid.
Sure, maybe we can attribute this to Magic Man Orpheus and his aura of BS. But at the very least it's indicating that these tadpoles are somehow (And for some reason) causing their original hosts minds to be kept intact and then suppressed by their Illithid selves. With this suppression being able to be itself supressed by Magic Man Orpheus (Though, somehow selectively... As other Mind Flayers besides the Emperor are unaffected. Not to mention you can waltz right into the middle of Moonrise and no other "True Souls" find themselves shielded by the artifact you're holding...)
