My daughter is one of the people that has only one identified mutation- df508. She has a clear dx of CF, though, because of her symptoms: sweat tests of 90 and 108, severe growth failure followed by dramatic response to starting digestive enzymes (more weight gain in the couple months on them than in the previous 3 years!) and even though she doesn't have, to date, severe respiratory involvement, she's had worsening respiratory infections each yr since she was a toddler.
One of my boys has one df508 mutations too and he is the one we are confused about, as he had two 'borderline' sweat tests and only mild symptoms that may or may not indicate anything worrisome (chronic sinusitis that makes him miserable if he doesn't take something for it daily.) Our next step is investigating to see if he has congenital absence of the vas deferens; if so it'd be diagnostic for CF rather than just being a carrier.
Interestingly enough, my husband's nephew with CF also has one known and one unknown mutation. It would seem that my husband's family is the carrier of the mystery mutation... hopefully we will be able to identify it someday!
I hope you get the answers you need. It's frustrating when it's SUCH an important thing to try to figure out and you know you need an accurate answer!