With "much adjusted" flavor I see it as what I said earlier, some sort of mixture of insanity and his latent fiendish heritage being combined and tapped as a source of power. However, I want you to be as happy with my character as I am when it is done, makes things more fun in the end.
Mechanically, everything that I really want out of it can be gotten with feats, mostly the immunity to mind affecting things (because I'm crazy). And I don't know why but when my characters never have to eat or sleep that satisfies some of my paranoid instincts. I can actually survive just fine without the PrC, and in fact will regain fifth level rituals if I drop it.
I'm just lazy and don't want to re-evaluate all of my feats to make them work.
Oh, question: Does immunity to all mind-affecting spells and effects include immunity to all fear effects?