If your chickens have canker, then meteonidazole or ronidazole would be the best treatment. I would cull hens with canker, especially if I had chicks coming up who would get exposed in the waterers or feeders. Once canker is in a flock, it will continue to spread. If you cull a sick hen, a necropsy with histology by the state poultry vet would tell you what exactly the hen had. Since you had coryza several years ago, other hens could still test positive for that because most respiratory diseases make carriers of the whole flock eventually. Those diseases keep getting passed on. You have really had your hands full dealing with the multiple problems over the last few years. I admire you for trying to learn what was going on and getting some vet help to deal with the issues. But when the state vet does a necropsy, they will usually test for the usual respiratory diseases, which kind of gives you an idea what might be hiding in the flock. The lesions you are dealing with inside the beaks could be canker, wet fowl pox, or candida (fungus,) but the necropsy could be of help.
