What a truly wonderful story this was. Utterly, utterly brilliant. I don't know where to begin praising it, so I'll just give you some of my favourite lines.
she hadn't once made mention of the obvious fact that he had been her student, which put her miles ahead of the rest of the staff in the mental tally he was keeping. She did not correct his grammar, as Binns did. She did not eulogise his dead schoolmates in his presence, like a certain headmaster. This is so telling. The difficulties of that first year teaching – which is difficult enough in itself, as every teacher knows, even without the added strains he had to endure.
pretend he had in fact been the recipient of a social invitation before. How bleak. And so very Snape-ish. As are his financial worries, and the embarrassment over his clothes.
"Why, Severus. How thoughtful of you." Your Minerva is a delight from start to finish, and this line is so perfectly IC. As is To that end, I intend to occasionally subject you to conversation.
Severus thought he could hear the faint pounding of the surf beyond it, but it might have been the throb of an impending headache. The way you describe the landscape and mix it with Severus's moods is – again, sorry for being boringly repetitive – brilliant.
"And what would I want to look at you for? The whole interaction between Severus and the ghost is so eloquent – their bickering, and the way Severus accepts her: as an old friend almost. He would; he's been nearly dead in his mind already.
"Like you know you should be sleeping but you aren't." He nodded as though that had been exactly what he suspected, and he put out the lamp, and he got into bed.
provided he doesn't curse a healer again." That detail is such vintage Moody – as is Minerva's unfussy determination to help him, nonetheless.
oranges, yes, and perhaps aniseed, and something more deeply herbal. I love the way your Snape, like the Potions Master he is, is always aware of the smells around him – and dissecting them, listing the certainties and possibilities of the ingredients.
The way their lovemaking proceeds, from student-teacher to equals. The comparison between Minerva and the sea. And the painful, needy path Snape and Moody take. That devastating 'Gideon'.
"I had a lovely holiday, Severus," she said, and then she opened her book. How enigmatic Minerva remains, right through the end. And the contents of Severus's satchel!
And what an excellent idea to have Minerva collect butterflies – more cruel a hobby than most authors would give her. But it fits so well with your version: someone who takes, as well as gives. Nothing remotely saintly about her, despite the help she gives both Moody and Snape, and the immense restraint she shows in not asking Snape about any of those bruises.