A) She said she waited from noon but nobody showed up. Why she hadn't left by the time Neville was there I don't know, but maybe she'd sought somewhere safe and the hubris attracted her attention.
B) I thought it was pretty clear that they did. I would have also liked their society to be explored, but at the same time, I thought it was really interesting how Neville misinterpreted the leader's actions in exposing himself to sunlight, and our failure to get more into it seems to be a parallel to his errors here. There were still strong hints of the book's original premise (ie (SPOILER)that Neville has become a legendary monster to those infected who retain intelligence and social structure, a factor which in the movie seemed a little more combined into a single being.) dotted throughout, which I personally found very well done. I don't mind that the movie doesn't follow the book perfectly, but I thought it paid appropriate respect.