Hi Sonia,
I've been reading your saga and hoping that you would feel better. I don't know whether you want advice or not, but I'll throw some thoughts out to you. Use as you see fit.
It's very strange to me that they give you a nurse to manage your case. You have a lot of questions and if it were me I would insist on seeing a doctor. This is CF, not a simple cold.
About your exacerbation... is it possible, since you cannot cough up anything after just a week on 2 antibiotics, you are improving? -- Even if it isn't as quick as before. With each exacerbation, we have more damage, more inflammation, more scarring. I know that as I've gotten older, I am so much more sensitive than I used to be. And exacerbations take new and different courses than before.
About cultures. Considering in Sept. that you did give them a good sample that cultured something, the cultures do identify which drugs to use and how effective they will be. If you look at your culture, the numbers next to the antibiotic (eg. >8) are indications of that sensitivity. So the 2 drugs chosen for you should work if in fact you have an active bacterial infection.
By adding a third drug or fourth, especially if there is no evidence that they are absolutely needed, your bugs are only going to become resistant to more and more antibiotics very quickly. Then what?
A word about PFT's. Treating an active infection usually leads to PFT's going up. But if your lungs have a lot of inflammation, the PFT's are not going you go up as quickly as you'd like. Adding another antibiotic won't help this. Inflamed airways could lead to subsequent infections. That is one reason some people have endless cycles of IV's. The bugs that aren't killed off, stay trapped in the narrow airways and infection starts brewing all over again.
If you have inflammation, unless it is managed, it is going to be very hard for you to get air in and out of you airways-- resulting in low PFT's. A possible cold, just adds to the problem. Discharge dripping down causes airway inflammation too.
Maybe I am way off the mark. In any event. I hope thay you are feeling better soon and back to your old self.