Excellent story, absolutely one of the best. I have read this once before but it was just as addictive this time as it was the first time.
BTW one of the many perhaps minor things I appreciated about your story was that you did not turn Snape into a mono-dimensional evil character. He is, to me anyway, one of the most fascinatingly complex characters in canon, but so many fanfiction authors, in my experience mostly young male authors, hate and 'bash' him into a boring cardboard villain.