I love the way you describe the settings and Minerva's vigilance. The gradule way that you move from Minerva's sincere apology for her doubts and mistrust to the revelation that they once had different relationship. It's funny but I've never been able to "see" Snape / student or even Snape/ former student. Those stories always read like he has a personality transplant to me. But this feels romantic and right even though he once was Minerva's student. I guess it's because I don't think she was casting lustful glances toward teenage Severus! You've painted the idea of a gradual relationship between two private but passionate adults with much in common outside of their personalities.
Shaking his head wearily, Severus continued, "My obligations are finally at an end. I'm a free man for virtually the first time in my adult life. I've paid my debts as best I could, taken responsibility for my mistakes and tried to make up for them in the only way I knew how. The scales of my life are as balanced as I can make them. At long last, I can look to the future, now that it appears I actually have one, which is something I never even dared to dream about for a very long time.
"So I've spent a lot of time over the past few weeks, looking back, sifting through my life and wondering what on earth I was going to do next. And as I was thinking about the past, I came to realize that the times in my adult life when I've been the happiest were the times I spent with you." His lips twitched ever so slightly. "No one has ever been more fun to argue with than you, Minerva."