This is a great story! I have to admire Tonks' honesty when she finally gives the parchment to Filch.
I think you're right on the money with your characterization of Bill. Molly would have put the fear of God into that boy because he's her oldest and needs to set an example for his younger siblings and also because she has high hopes of him becoming Head Boy.
Finally, the scene between Tonks, Sirius and Remus made me smile... Dumbledore's theory that there should always be a little more love in the world and all that. Well done.