Wow... that was really amazing. One of the reasons I don't read a lot of Snarry is because most in most fics, Snape gets really OOC throughout the course of the story, and by the end, you're wondering if it's even Snape at all.
I love how he's snarky and rude and believable through the whole story, and he changes naturally due to the circumstances rather than because the writer wants fluff.