Heh, I'm glad I read the fast-and-loose-timeline warning, because even with that, I did have a wee moment of dislocation ("wait, how young is Severus supposed to be?") with Lucius still around :).
I'd agree with the comment that described this as more retro Snape/Black - it's hard to see their characters in here from the more recent canon, but it does fit with the softer approach that earlier stories often take.
Snape's relationship with the unicorn was both sweet and poignant, and the story by turns cute, funny, hot, and sad :).