I love the backstory you create for Severus - very original, from what I've seen, and it fits with a lot of things. His constant testing for ulterior motives and intrigues is exactly what one would expect of him, and you really allow us to feel his isolation under all of that wariness.
Yaxley is intriguing too, here - thoroughly Slytherin and I like the idea of the pureblood dark/pale dichotomy as one of the fault lines which runs through this world. His pragmatism is almost refreshing after all of the backstabbing and power-plays amongst the Death Eaters and it's hard to fault his admiration of Severus. :-) This was a pleasure to read. Well done, Mystery Author!