Hello
I'm sorry I don't have any specific advise but I thought of this - maybe try contacting some Yorkie rescues to see if they can advise you. They would have plenty of experience with helping an adult dog who has been neglected be rehabilitated to fit into a new loving home. I'd imagine you should be able to find one to help you since your case is really something that they deal with all the time and helping dogs in these situations is what their passion is
Good luck