I have followed your story since I started on this board, for totally unrelated reasons I was looking up glutathione and n-acytl-cystein and noticed that NAC is a mucolytic and I thought of you. I have no idea whether it might help, but it might be worth exploring and it is different than anything you have mentioned trying. Several of the websites mentioned that NAC is very safe, they use it for Tylenol overdose in hospitals, and in the US it's available over the counter. You can get it in a nasal spray too.
Just a thought.