Recent experience with my own bumblefoot and now a foot infection for Romy backs up Miss Lydia's suggestion about Epsom salts. I made a poultice for myself with Epsom salts and comfrey salve, and for Romy we are putting warm Epsom salts/water compresses on twice a day, along with a triple...
Might an Epsom salts compress help her make progress?
I have heard of DMSO and hesitate to try it because it can be really potent. Glad to hear Miss Lydia has had success with it.
I think Epsom salt soak is definitely worth a try. And yes, I would ignore that vet's advice. The penicillin idea also sounds reasonable to me. You have just started working with her. Stick with it, sometimes we lose them, but many times we do not.