There were two things that i especially liked about your story, considering them quite ingenious...first off, Snape's animagus form. :) Usually he transforms to bat or snake, but a scorpion - much like Snape - has venom and powerful claws, but still can be so vulnerable if cornered. (I sound like Hagrid, i realize :D) The second thing also relates to the plot. If someone chooses a plot-line similar to this, meaning: everyone thinks Snape died, but in reality he did not, well... usually Snape just 'reappears'. I mean, he reveals himself and stands for example in front of Harry in all his human glory. Very dramatic, if written right, but somehow, I prefer your version. Plot-wise, it was very well thought-out. :)