I have never heard of "half illihids" in D&D...ever. Illithids conduct all sorts of mad experiments with tadpoles sticking into every creature they can resulting in all kind of abominations like mind flayer dragons, elder brain dragons, etc. But generally speaking I've only ever heard of two possible results: a) ceremorphosis completes and some new mind-flayer or ulitharid or some other abomination is created depending on the type of creature the tadpole was inserted into OR b) ceremorphosis fails and both the host and tadpole die.
So I haven't the slightest idea what a half-illithid would be or how it would work in relation to elder brains and mind flayer colonies. But it sounds like fun.
The Aberrant mind sorcerer is essentially half Illithid other half other creepy psyonic power of your choosing.
