I've got nothing on the Mometamax... questions about the face issues, though. Would you describe the spots as "crusty?" Does the spreading seem to be symmetrical at all? It could be pemphigus, which is an auto-immune condition in which the immune system goes wonky and starts attacking the "glue" that holds the outer layers of skin cells together. You would diagnose with a skin biopsy (which can provide you lots of answers as to what it is or isn't) and treatment traditionally has been steroids, but we have had great success with Willie on cyclosporine, an immune suppressant drug originally developed to prevent organ rejection in transplant patients. "Crusty" sores that spread in a symmetrical fashion are hallmarks of pemphigus, and the sores can appear anywhere, but are common on the paws, face, ears and the base of the tail. Willie's first presented on his belly and eventually affected his entire underside, his head, face, ears and front paws before we got him diagnosed and treated.
I'd recommend a skin biopsy if you can swing it. Even if it doesn't provide a specific diagnosis, it will quickly narrow the field of possible causes so you can better focus diagnosis/treatment options.