This is great -- you accomplished so much in a relatively short fic. I liked Harry briefly covering for his move towards commitment with the insinuation that he's being a bit of a good-time boy, making out with Lupin and letting the older Weasleys grope him. But of course the connection with Snape comes through.
The idea of tying the bond to the Tri-Wizard Tournament makes a great deal of sense as well.