Thank you all so much for the help, especially dawg53 who has been trying to help me figure this out from the beginning. But I'm afraid the mystery of what's ailing my sweet BR bantam will never be solved. I did give the ivermectin, and I put her back with the others last night and things went fairly smooth when they woke up this morning. But her episodes of gaping, head shaking and scratching have come back full force. I wonder if it has something to do with being outside? When she was in the house, it happened much, much less and I thought it was because of the antibiotics, and maybe it was, but it seems odd that once she went back outside it all came back. Can chickens have allergies? Could that cause these kind of symptoms? I know I'm grasping at straws but this is so frustrating. I've been trying so hard for so long to make her better (I won't even say how much I spent at the vet) and still no answers. I also still wonder how these symptoms relate to the fact that she doesn't lay anymore. She's in her prime (1 year old), but she stopped laying months ago when the gaping started.
The gaping, head shaking, scratching fits seem to come in episodes and make her really uncomfortable for a while, but in between these episodes she's a normal happy girl. So I've decided that as long as she's thriving, I'll just let her be. I'm not sure what else I could do anyway, I've already tried every avenue available. She was so glad to be back with the flock scratching and dust bathing. If she ever acts miserable or sick I'll try the vet again, but I don't think I'll ever figure out what's causing this. So as long as she's happy, she can live her life like a normal chicken. She's a sweet gentle natured girl and she deserves that.
Thanks again for all the help, if anyone has any more ideas please let me know. And I'm very curious about whether chickens can have allergies and what the symptoms might be, if anyone knows.