Remus did kill Greyback. And yes, Severus got Rodolphus. Never fear, we'll find out more, probably some in the next installment, but definitely in future parts. ^_^

And yes, I'm with you on the physically frail thing. For one thing, he does an awful lot of dashing about in the books, when people are threatened, and he also jumped between Harry and Lucius in the Department of Mysteries. I'm pretty sure he might be tired and run-down by the transformation sometimes, but he's not frail. ^__^