if you present with a normal or borderline (more likely) sweat test and one gene but are symtomatic then you can be diagnosed symtomatically. This was the basis for my diagnosis because I was only showing one gene. A few years later is when they finally started testing for my other gene in the full panel.
I was diagnosed as an adult but relished the diagnosis since I was feeling poorly nad finally had an answer. That is not to say it was not hard.
From an outsiders perspective here is what I woudl do if I were you:
If you are not already seeing a gastro for the belly I would do that, CF docs focus mainly on lungs and need to refer out when their are problems outside of the run of the mill stuff but some are hesitant. Also go to a health food store and read some articles talk to a consultant, there has been alot success helping iwth stomach problems thorugh homeopathic remedies (not stopping regualre treatments).
Seek out a counselor whether it is at a church or thorugh medical community. Late diagnossi is alot to deal with.
Do you do xopenex or albuterol before your treatments? I rarely cough up without it. Some CF'ers are not as mucous producing as others. If you are not culturing any serious bugs then you might not be producing much. What are your PFT's? Why are you gettting tune ups so frequently? if your numbers are going down then either there is a build up in there forming infection and you are truly not getting it out or maybe you have bad ashtma and that is not being addressed? Do you excercise? If I do straight physio I am about 10% as productive as if I combine iwht excercise.
If being treated as though you have CF (would be more aggressive) is still not getting your problems worked out then my guess would be acting as if you dnt have cf and tryign to get treatment would be less effective.