I can't help but think that the success of this 'defense' depended on the fact that Snape would regret Dumbledore's death - which meant that whoever set it, believed that Snape did not mean to kill Dumbledore (at the very least).
Lemondropseven, from you description of this ficlet I got the distinct impression that you saw the DH part 1 movie: (I liked how this scene was done in the movie you said above): I would like to ask you (if my answer includes a spoiler, just post your answer to my journal): Does DH part 1 include the Snape/Nagini scene? If not, where is Snape at the movie's end?
I am asking this b/c I don't want to get a nasty surprise just by going in to see the movie.
Lovely ficlet. I particularly love how quickly Snape figures out that he still has the 'nonverbal' defense. (Even though, personally, I think Snape went to Grimmauld Place in order to get ahold of Harry and impart something to him.^^)