Lupin's writeup (I'm assuming based on what the ISS told him), says that "Having subdued all but a handful that managed to remain hidden, Crouch and Dolohov killed Chloe Selwyn and then inexplicably left".
I remember wondering why they killed her when I read that--it made more sense with Dolohov's post this morning.

On Barty: I'm not sure, beyond 'Tosha asked him to' (I'm fairly certain he knows that Tosha knows more than he's saying--and there I go changing what I'm calling him again--it was fairly obvious, after all--but I'm not sure what exactly he thinks Tosha knows.)
I think that might have something to do with wherever they went when they left for a bit.