It could be canker / trichomoniasis, does his breath smell bad? I've had a couple of cases and have successfully used flagyl / metronidazole in conjunction with nystatin in case there was also a yeast problem.
Even if the vet won't treat your chicken, tell them that she has a respiratory infection and needs Baytril. Unfortunately a vet prescribed antibiotic only. Sounds like you are from NZ, so probably the same there.
Sorry to hear about the renewed infection in Mildreds foot. Really sounds like she needs oral or injectable antibiotics. Any form of penicillin would be good, have you checked with friends or family to see if they have any leftovers in their medicine cabinets?
If you know anyone with livestock...