Not a veterinarian, but this looks like Aspergillosis. It's a fungal infection which would explain why none of your attempted treatments helped.
Unfortunately, there isn't much that can be done to treat aspergillosis. The most important thing you can do is ensure your birds are coming from a clean source and you keep them in a clean, dry environment.
My thoughts were possibly Mareks disease or other viral tumors, but I wonder about fungus, such as aspergillosis or histoplasmosis. Your state vet pathologist might be of help if you could send in some tissue.