yeah, rotate her body so she's "cheating" to the camera in a 3/4 shot. another trick, besides what you mentioned, for reducing the under the chin thing is to have her press her tongue firmly against the roof of her mouth. that tightens the muscled under her chin. try it. you'll feel those muscles under her chin to tighten up.
as for the available light thing, you kept the sun behind her... that's a good thing. get yourself some white board and mount it on some styrofoam or get a collapsible photo reflector and find a way to set it up to bounce some fill back on her. better yet, get someone to help and have them bounce the fill light in with the reflector.