have girls already been chosen?

Every sorority has a list of pnm's that they are interested in way before rush. The sorority's return rates determine if they actually get those girls because the pnm's who have choices may not return to a house that is interested in them. And sometimes the pnm's on a sorority's 'list' show up with bad attitudes and they are not asked back. Nobody's "chosen" until the final bid list; there's always movement during recruitment.

To an extent, yes. Every sorority already has some girls that they know they want (“top girls”). They will prioritize these girls during rush. They also have girls that they want, but are not fully convinced of bidding yet. During rush, the actives will meet these PNMs and decide whether or not they want to bid them.

A lot of it depends on whether or not the sorority thinks the girl will want them. If the girl is known to associate with girls in a certain house, it’s likely that she would get dropped from other sororities, but not always.