You nailed the 'canon' prompt, that's for certain, and the infidelity one as well. Using dialogue almost exclusively to tell a story, and tell it well, isn't a feat that many authors can accomplish, but you have and then some. You moved the reader through decades with only a handful of scenes, and yet you chose them and fleshed out Harry and Snape so well, that the passage of time seemed right and seamless.
What an ingenious way to throw Snape and Harry together, and still keeping them in the 'reality' where they belonged. Lovely love story.