There are plenty of things to enjoy in this story, but my favourite is probably the differences in the ways Severus and Minerva think - how they interpret things differently because of their respective pasts. They both have their sensitivities - Snape to being treated with condescension as a Death Eater, and McGonagall to being treated with condescension as a woman. And if their Slytherin and Gryffindor backgrounds sometimes lead to misunderstandings, the things they share - like common sense, respect, and the occasional (healthy :D) criticism of Dumbledore - are a solid foundation for friendship(+).
I like your take on Snape here (on Minerva too, but I've always considered Snape difficult to ship whereas Minerva is just more approachable as a person): he isn't romantic, but he gets the small things right and he is someone you can rely on.
Last but not least, I much enjoyed the touches of humour :D. This was great fun to read! Thanks again!