This is full of things I greatly love: Severus surviving DH, Snape being super snarky, fellatio performed on Snape, wanking, and very hawt p0rn, especially when Snape gets all desperate and incoherent.
I really like your version of what happened after Snape was bitten by the snake a lot. It's very original, as far as bringing in the Ministry, Madam Pomfrey, and Narcissa.