For me, the star of this story is the Scotswoman, and your IC portrayal of both Snape and Dumbledore. Two prickly, lonely men who are so ignorant of social niceties, how to give, and how to receive. My heart sort of broke a bit, that first time when Snape left, but it was so well-done, that part. I wouldn't have bought it if he'd stayed. Snape doesn't seem to be the sort of person who'd do anything impulsively; he'd have to agonize over it first, and I'm so glad he went back in the end.
Thanks for writing this pairing; you did it so believably and well!