I think you caputred the characters and their motivations perfectly and, other said it better than I ever could, tie JKR's losse strands. That part was really enjoyable to read, after...
Well, I am the last to be picky and my own use of the English language (as it is not my mothertongue) is nowhere near perfect, but especially the first part is riddled with typos. No real mistakes but things written together or jumbled. The second slight complaint I want to make ist that, even if your perfekt handling of the charakter's natures involved, the storyline itself leaves me kind of cold. I cannot really express what it is exactly that made me frown but, for me at least, there was something missing. Your story isn't action packed, that's not in its nature, but there was no real deep emotional excitement transported to me, the reader. Not about making peace with Dumbledore, not about the occuring deaths. It might be that PaintedSnape simply lacks the fullrange temper and feelings LivingSnape would express, but- maybe the best analogy would be reading the Illiad aloud in the same tone of voice? Or it might be just my own failing at 'getting IT'