Well, the kind of yeast to use is not baker's yeast, it's brewer's yeast - that is important. Don't want to make the little sick. I am not sure what you have is brewer's or nutritional yeast. Just trying to avoid problems.
Epsom salts can get into the tissue and help the healing process, even once a wound has healed over. Serious injuries can take quite a while to heal, and there may be residual damage, but ducks are so very resilient, and I would keep my hopes up. Keep up with the hydrotherapy (bath time).