Well, fuck. Severus is definitely a sadist here--which I really can see (as much as I like prefer to bury the darkest parts of his character). I think the most amazing thing is Hermione's strength--he never breaks her. Like you say not necessarily safe or sane but they both seem 'happy' with the relationship. (And as I am not crazy about this ship, I found it pretty hot that it was a bit dark rather than reading like a romance novel.) Well done.