Given your propensities as displayed in other fictions, I was expecting . . . well, shagging.
I'm a Snape/Hermione fan, and one of the things most delightful about that pairing is fanon Snape's ability to be smooth and witty, sleek and powerful in his dealings. Snape/Hermione fanficcers have a tendency to display Snape as someone a lot happier, and under a lot less stress, than he really is.
One of my favorite things about Eliot's poetry is the sense I get in it that this is what I'm missing--THESE are the things grown-ups talk about after the children go to bed, the things that make them grown-ups and not just adult people. This, too is what I imagine the conversations between Order members are like, at least in those dark watches of the night when they're talking about what's really on their shoulders and not just Fighting the Good Fight.
You've captured this sense perfectly: not so much what makes Snape Snape and what makes Lupin Lupin, but what makes Lupin Lupin when he's by himself and not being reassuring for Harry; and what makes Snape Snape when he's not at work. Snape's kindness comes out so barnacled and rusted from disuse that it is scraping to him and full of stops and starts to everyone else. Lupin's sadness comes out as a grey and overwhelming wall of regret and of uncertainty as to whether or not they'll win this thing, and forgetfulness as to what, exactly, they're fighting for. This, I think, is the way Snape and Lupin--and SnapeandLupin--would be as themselves, not in the hands of fanficcers or of those who haven't gotten used to their adulthood, or gotten to the place where they have these conversations after the children have gone to bed.
The only other thing I'll say about the fic is that I'm normally opposed to graveside scenes, usually because they seem out-of-character. This did not: the solemnity with which Snape conducted himself made the difference. The sound of his voice in a secluded spot reading Eliot is a stark and appropriate image.
Thanks for writing. I appreciate it.