I don't know anything about mouth sores, but I'm sorry to hear about your kitty. Did the vet think she was allergic to a certain kind of food (like chicken) and prescribe a particular other kind? Did you first take her to the vet because you noticed a sore, or because she wasn't eating?
I don't remember what depo and convenia do, though my kitties have both gotten them, so they must be fairly generic and not specific to kitty's mouth sores. Other people here are much more knowledgeable than I am, so I hope someone will be able to help out.