Hmm. Well, I'm not sure I'd say that Nick is a Stu. My understanding of "Stu" is that it's a character who is really a surrogate for the author her/himself. I don't get the sense that this is what is going on here, at all.
He's not annoying, and you finally did give a semi-plausible explanation for what all the mystery around him is really about. But if he inherited so much money, I'm not sure why he'd act as a defense attorney for organized crime, so I still think he's lying.
But I think you're developing the relationship between him and Harry believably (from Harry's POV---have no idea what is going on in Nick's head). Harry's reluctance and conflict re the gifts and money is done very well here. And his relative lack of caution in this relationship is also believable, given that it's his first relationship and he's young. Overall, I find their interactions to be plausible, and Nick's not annoying. (I mean, someone that manipulative would be an annoying lover in real life, but he's not annoying to the reader as a character, if you see my point).
BUT. I just don't see where this is going. How are you going to get Harry to transfer his affections from Nick to Severus? If you want to make the drama revolve around the idea that Harry falls for Nick hard, but then is torn by his (unacknowledged) love for Severus, and has to choose: well, that could work well, but you'll need to build up the relationship and love for Nick even more, alas. Right now, I don't see Harry as being that torn if he has to leave Nick.
On the other hand, if it's NOT supposed to be a wrenching choice for Harry, then you may have built up the relationship with Nick a bit more than you needed to.
Sorry not to be more helpful. I'm rambling, at this point. I AM enjoying this, however.