Hi Ken,
Bummer of a case. We use Hoxsey for severe stomatitis. Mona can give you a homeopathic version that might be easier to give.
For severe oral pain, I'd wonder about homeopathic Belladonna over Arnica. I don't know if you're a member of the CAVM list, but if you are, there are homeopaths there that can repertorize the symptoms for you if you don't have those books yourself. I guess my top pick of a remedy might be Hepar Sulph. Lesions are very painful and septic.
Hope you have some success soon. We're there with you,
Steve