A mortal brain cannot defeat the elder brain as ithat entity can process all possible moves at once. It is always ahead of you. As the emperor explains, only an illithid brain has a chance to match the speed of thinking to make a move before the brain has a countermove ready.
Furthermore the one who combats the brain also needs to control the protection of Orpheus. So therefore the combination illithid power + orpheus power combined in one, is a prime requisite to move against the brain. So we canhave either Orpheus becoming illithid or an illithid acquiring Orpheus protection power.
I don't know how this works with Tav taking the illithid role though as I never tried that (and don't want to) and a less likable NPC, like Gale for instance, is not allowed to change. So if Tav can turn without eating Orpheus' brain then the story breaks down.
That doesn't make sense, Elder Brains are killed all the time by mortals, without the need of an ilithid traitor.
Neither do have elder brains some special hyperintelligence. They are very intelligent, but so are dragons or even some mortals and no one would buy "to defeat a dragon you need a dragon".
Also "you can't outsmart a bullet", or in the case of D&D a sword. When you are hacked apart with magic weapons as you are big and immobile it doesn't matter if you can think a bit faster than your attacker. You are not playing high stakes chess.
