My DD's old gym had open gym but it wasn't anything structured and was often run by some older girls (upper level) who were paid to "play" with the kids. One regular coach would be there but most often, not even on the floor. My DD only did open a couple times because she was already doing about 12 hrs/week so I wouldn't let her. It was playtime and she would make up her own floor routines, swing on the rope, sometimes older girls would help her on strap bars. If she went to do something there would always be followers but for the, most part no bullying.
I would say something about that to the HC. I probably would have ran out on the floor myself if I saw my DD being bullied like that. Maybe if there are alot of kids participating they could structure it a little more & make sure the hands to yourself rule is being followed. About the coaches, it usually is more of a playtime and I can see why they may not enjoy it. Maybe you can try another area gym to see if it is structured differently or has less participants. Sometimes you might get lucky and find a coach who is willing to work on some "serious" stuff in open gym, but most of the time open gym is used as a "make up" for missed rec classes and many of the participants are either rec kids or kids that may be looking into joining the gym.
Good luck!