Ea R'lyeh Aboleths.

[Carcaroth]

I'm certainly not Kristoffer, but I do think the most interesting way to handle it would be a series of holy spells that transform your freespawn from the mothers/that one pretender into more specialized unit. I don't know about the grafting idea, though it would be interesting if it could somehow be integrated as an in-combat secondaryeffect with MR to resist, but the enslavement and multi-phase of life thing makes me think the spells should all create, well, more aboleths.

If one pulls from DnD, particularly the really entertaining Lords of Madness book, there were flying and Stygian varieties of aboleth, and given the good weaponry/stats basis, you could probably do quite well just working with them as a chassis. Even the Abomination could quite reasonably be considered an aboleth variant.

[HoneyBadger]

Well, I've thought about summons that gave you versions of the Mothers which would produce new types of spawn, but to be honest, I was planning on using that particular idea for a middle age Aboleth mod nation. Also, undead Mind Lords.