I am so sorry you and your daughter are having such a rough time. Reading this is breaking my heart, partly because my son is 14 as well so I know the age well.
We are very compliant in our house. Andrew might miss a treatment here or there if he gets home late from a game, etc., but I would comfortably say he is about 95% compliant. So, I don't have alot of advice for you in helping your daughter help herself, other than to offer that Andrew has enjoyed watching some videos online from people like Gunnar Esiason and Ronnie Sharp(run, sickboy, run), about growing up with CF and doing treatments. They are role models for him. (And good ones, too!) There are some great young women who share their CF stories openly who might be a good fit for your daughter, too!
But my main concern with your post is your daughter's FEV1. Honestly, I don't think fev1 in the 20s and 30s is solely the response of poor compliance. May I ask how long her lung function has been that low? What is he lung function typically? It sounds to me there is an underlying issue, and mycobacterium may be the culprit.
My son grows mycobacterium abcessus. He has seen drops in his fev1 of up to 20% and has been on ivs on and off for a year and a half to treat the bug. He has rebounded quite nicely, but we are aware that m. abcessus rarely just 'goes away' and that we will probably have a need to treat it again. I would ask the doctors as much as you can about the mycobacterium she is culturing, what kind it is, and what their plan is. I would request a CT scan, as m. abcessus infiltration is quite visible on scans and can give a better idea of just how much infiltration has taken place.
Perhaps if your daughter has been feeling progressively worse, nebs and therapy don't seem to be helping her and doing treatments seems like a waste of time. Maybe if they can treat this infection and she starts to feel a bit better her outlook would change.
Please feel free to message me any time. I will be thinking of you both.