This is so nice. Elegantly written, elegantly constructed, and I love that you make Snape a Sherlockian without going into any further efforts to stress their resemblance. I also love how they had to fumble through their new roles, and the photo, then the letters are a perfect way to tie the whole thing together.
Bonus points for Catullus. ♥ As a long-ago Classics major, I can't read Catullus without thinking heterosexually as Lesbia, aka Clodia, was so thoroughly 'The Woman' for him - but I can well imagine both Watson and Holmes having learned the poems in school. Having them express their love in Latin verse seems very IC! What a lovely touch.