This is pre-slash, isn't it? I'll go on thinking it is, someday far in the future. Snape-Potter interaction rang completely canon and believable to me, all the way through. Sixth year progressing differently is always gratifying to see. Nice story, and points for reasonable actions and explanations for everything, and a canonically shady Dumbledore. The only issue incongruous with the original is Snape's ability to apparate within Hogwars.