You've captured so much of what I like about this pairing: the realistic awkwardness of their first time (I'm so glad you didn't go for "romance" perfection), the fact that so much is unspoken between them, their mutual reluctance to use the "L" word, their competition and banter, the tenderness beneath it all.
Other things I love:
--how pragmatic and in-control Minerva is; I love when she's crisp and confident and unafraid of her own wants/needs --Andrew Marvell! --your backstory for the reason that in canon, Albus's office won't open for Dolores. Of course Albus and the Heads of House would be on the job beforehand. (Sounds like a wizard rock group -- "Albus and the Heads of House" /g/) --Severus unpinning MM's hair (I have such a long-hair kink, mmmmmm) --the fun way you play with that fanon trope of Severus's buttons --how well Minerva can read Albus (and the fact that Albus is unaware of her relationship with Severus) --savvy Filius. He's not a Ravenclaw for nothing, that boy. --how well everything fits in to canon --the fact that you don't ignore the age difference --Minerva's realization that she's been too indifferent to Severus in the past
Lines I like:
She was not certain when she had come to care for the man sleeping beside her. Perhaps around the time he relaxed enough to fall asleep in her bed. This is so "them." Evenings sometimes allowed an enjoyable discussion with Filius or tea with Pomona, but more frequently meant drama in a castle filled with teenagers and late-night skirmishes with Severus as he marched into her rooms. This sounds exactly like what I'd think a kid-packed boarding school would be like. And of course the Severus-skirmishes /g/. She had spent enough time around Slytherins, however, to hear the unspoken question. Yes, so much of their interaction would be indirect. She had seen enough damaged men to know that he was probably not capable of a healthy relationship I like that neither of them wants love/romance at first; they're both such pragmatic sorts that I think a relationship of convenience makes a lot of sense. And I like that she doesn't have illusions about him. He was rather impressive when he got going, she thought. Ha! Yes, indeed.
Thank you so much, Mystery Author! I think you must know me -- you've hit so many of my bullet-proof kinks.
(And I'd be sad at the end, knowing that time's winged chariot is drawing ever nearer...except that Severus doesn't die, of course. So they DO have world enough and time.)