Oni: Thank you for posting your objections to the Incanatrix PrC. That's what I meant by "rationale."
As to your points:
-IMHO, Iron Will is a burn feat for a class with a good Will save. But maybe that's just me.
-A sorc is only getting access to the Instant Metamagic ability twice per day, that too at 13th and 15th character level minimum, respectively. Is that really broken?
-Improved Metamagic is THE good ability for the class, and it barely "improves" anything at all, given how suboptimal a choice metamagicking is in general. The sorc especially gains little by this, since s/he has to waste a FRA regardless of the feat used or the level cost employed. I agree that the one big problem with this ability is its synergy with stackable Empower Spell, but then I don't think that feat should be stackable in general, especially since the problem comes up again in epic levels once Improved Metamagic is available as a feat.
-Yes, the class gets too many bonus feats. See my post above.
-Note that I agree that the class is overbalanced. It probably is the best arcane spellcaster PrC out there, largely because most of those PrCs are underpowered. The Arcane Trickster is better, the Fatespinner is probably better, and the Geomancer is more prone to serious abuse, but yes, the Incanatrix is seriously good. That's why I do agree with you that there are needed fixes to the class; I just a) don't think it's "one of the most broken PrCs I've ever seen" (pop open S&F, MotW, or OA to see a big grouping of those); nor b) do I welcome a response that has nothing constructive to say (your first post). Thanks for posting a well-thought-out response subsequently.
And yes, familiar advancement is NOT granted by any PrCs unless expressly stated in the class description (for example, the alienist grants this).