Oh, I adore this. I love the way the narrative fits into all the nooks and crannies of canon - it's excellent to see a fic set among the staff at Hogwarts while Harry et al. were off camping... ;-)
Your characters really lift off of the page - a strong, competent Minerva who really cares - both for the cause and for other people - and a Severus who commands all the sympathy he deserves for his awful predicament. The sexual dynamics here are exactly as I like to see for SS/MM. You have written virgin!Snape wonderfully, and the sex scene was both hot and heartfelt.
Also, what a superb counterpoint AD/PB make to the 'serious' drama! (Being a portrait might be dull at times, but it also sounds fun...). I particularly liked the 'voice' you gave Phineas - suitably old-fashioned, but also a little down'n'dirty ;-)