Hi, Daniele, several, but I hope you won't be offended!

Here are your remarks, followed by my comments after the asterisks:
"First, it is by definition difficult to access remore areas of the CF lung (especially those with mucus plugs in mid-size airways)."
*****That is a problem with everything you inhale in CF; however, GSH will thin the mucus in the lung, which will aid in penetration.

"Second, GSH is stable only in acidic conditions and you do not want to inhale an acidic preparation (it is pro-inflammatory)."
*****Your LUNGS are slightly acidic. You are inhaling a slightly acidic preparation into a slightly acidic environment.

"Third, GSH is highly reactive and if reaching oxidized regions, wil be instantly consumed and turned into its oxidized form GSSG (there is no known way to return this back into GSH in the extracelllular milieu)."
*****This is EXACTLY what you want. You want it doing its job--which is to reduce things and become oxidized. (Actually, there is a small pathway in the extracellular milieu to reduce oxidized GSH, but it is not a major factor.)

"On the other side we know that there is no problem with GSH synthesis in CF and we know that NAC is correctly synthesised by liver into GSH. Than we know that there is high consumption of gsh and assuming NAC it can feed the body with the necessary cysteine to make GSH and get it into blood neutrophils before they reach the lungs. "
******Cysteine in high amounts is toxic. That is why the body immediately tries to turn cysteine (from NAC) into GSH to store it. Your liver will try to give up 50% of its GSH to feed GSH-deprived areas in the body. But this was meant to be an short-term emergency response. In CF, it becomes a chronic response, with the result that the liver is severely compromised by trying to give up its GSH to help the rest of the body. Help the liver to conserve its GSH by providing GSH exogenously to the body.

Now, we use NAC with GSH in our family. But we make sure that the GSH predominates and that we do not give a too-high amount of NAC. If I had to give one up, I'd give up the NAC, because when the GSH is eventually broken down by the body, its components--including cysteine--will be recaptured by the cells of the body.