I like the new changes. It's much more balanced without Sadism/Masochism, though I admit I love the flavor of those two spells. Add in some more fluff if you want to emphasize the disturbing aspects of the Cult of Belial (which is, indeed, seriously disturbing), and I think a table for Affiliation Specifics in the Cult of Belial (like in Complete Champion) would be an added enhancement to the prestige class.
Great job! If I get the opportunity to play this class soon (which I'm currently lobbying for in my gaming group) I'll let you know how it functions in play.
Edit: I agree that the capstones are underwhelming at first, but the idea of suddenly turning an enemy into a friend in combat (especially with a Diplomacy check, which is easily boosted) makes me giggle with delight.
To boost power, you could change the effects to give the benefits of a Dominate spell, or boost the duration.