D.C.
Ah, one of my best friends has this. I know for her, she has to avoid things that cause inflammation, because that exascerbates it. She follows an anti-inflammatory diet - no red meat, and even chicken or pork she only has about once per week. She's basically a vegetarian, because when she eats meat, her condition flares up. By watching her diet closely, she rarely has flares. She does occasionally still have one if she's coming down with a virus (because that obviously causes your immune system to rev up as well).
So, you might ask about preventing flares with diet.
I'm not sure about her challenge with diagnosis, because she's had this as long as I've known her.
Good luck, and if your daugher is like my friend and can get rid of the pain by simply avoiding meat, it would be worth it. I hope you and she will try it.
ADDED: I think my friend has the hypermobility variant. She doesn't have skin issues, but when she's having a flare-up, her hips will literally fall out of their sockets and she has to pop them back in. Sounds very painful. But the good thing is that most of the time, when she's not having a flare-up, she can do anything anyone else can. I actually met her in college when she was teaching aerobics, if that gives you an idea of how active she is!