1. Not that I'm aware of. All club activities are fixed as part of the main story. You will get involved with some other clubs, though, depending on the choices you make.
2. Of the two other girls in the gallery, Hitomi will show up eventually in the club storyline, and Azumi has her own interactions that begin when you move into the larger apartment.
There is one more minor character who will send you some photos, then will become available for a bit of further interaction later on. That's all I've found so far.
There are some multi-part NPC stories, too that you might come across, including the x-ray glasses girl, the bully hunter, and maybe a couple more.