That was an interesting premise, and I loved the *very* grudging way both of them go into it.
I can't express how much I loved the dialogue between Snape and Shacklebolt at the beginning, and Kingsley's amazingly frank method of persuasion - "You're it." LOL. Snape was wonderful there too (and throughout) - sarcastic, but with a self-ironic touch, fighting, but not fighting too hard, proving that Shacklebolt and Minerva were indeed right about him.
And I laughed my head off over "This has to be the most unromantic seduction in the history of unromantic seductions." "We're pragmatists, Lupin. Survivors. We don't do romance." So brilliant (and true!). Though I felt that there was more than a touch of understanding towards the end, and I adored the tentative, uncertain ending - it fits the story perfectly.