@listenfortheloveOH god, I totally love your idea that Nezumi's patronus is a snake... and that it's of course just because of slytherin. Hehe! That's a really cool idea. ^^
And Shion's scars... totally an accident.
Ah, but do you know what's too bad? When Nezumi and SHion are in different houses, they can't share a bed together. I bet they always sneak out at night and fall alseep somewhere together. I could imagine that the invisibility cloak belongs to Shion. Maybe something he got from his dad. (I mean Shion's father made a cradle for him in Beyond. Maybe he left the cloak in the AU instead.)
Hm...Safu's feelings for Shion are also pretty strong. Maybe her patronus is a mouse and Shion's is a rat. o.o Ah... but well, most animagi may take the same form of their patronus... So Nezumi's should be a rat, since he's a rat then. But I like the snake idea better. >,<
It's fitting for Inukashi, though.
Ah, it would also be cool when Safu's patronus is something special, like a phoenix.