This is such an excellent story.  As for Severus being OOC--for the books, yes, he is.  But then again, in the books he's also a bully who holds a fairly vicious grudge against a son who never so much as knew his father; he may have reason to hate James Potter, but Harry did nothing to him.  For within your story, I don't think he's OOC.  I think that, by letting go of his hatred of James Potter and his resentment, he's actually maturing, and part of that process includes kindness towards a child even more abused than he was.  And I actually found it impressive that he seemed to put his concern for Harry above his own likely desire to tell Petunia what he thought of her.

So OOC, yes--for canon, not for this story.  And frankly, I think I like your Severus much better than canon anyway.