I haven't seen Grosse Pointe Blank in ages, so I missed a bunch of the similarities, but you definitely nailed the tone of the movie in this.
I was in stitches throughout the reading of this and thoroughly enjoyed it. I loved the pragmatism of Harry, the skills Snape has with the English language and his ability to cut everyone down to size, the whole conspiracy to get Harry to return.
This was so fantastic, right down to to the end notes.