Have you had a fecal done? Maybe she has parasites that are lowering her immune system and when she gets stressed or husbandry goes a little off, her immune system cant fight the RI and it takes over.

If it was me, I would raise the temps a few degrees, bump up the humidity some and talk to your local herp club, reptile animal rescue and zoo and find out who they recommend. That will kill some time for you to see if the wheezing goes away. If not, then I would call who they recommend and tell them the issues you are having and how you have gone to 3 vets already and they couldn't find anything wrong but your snake is still wheezing and it has bubbly saliva at the back of its throat and that he/she comes recommended from X. X being the zoo or herp club or reptile rescue.

I'm just not sure about the lung wash. It just sounds painful to me lol.