Crazy thought. If you can't get a statement from your doctor, what if you "sell" them back to the breeders. What I mean is, speak to both breeders and as painful as it might be, explain your situation. Tell them your reasons for having your babies and how much you love them, and then ask them if they would buy them back but on paper only, as long as some money is exchanged ($1) and it is written somewhere (a contract) it is considered a sell. Now you can "foster" those cute babies. They can live with you, and your ex can't go after them, they aren't technically yours. Get it?
I bet you anything your breeders as yorkie lovers will give you a hand. If not them then a family member or a really close friend. I'll do it if you want, as long as you take care of them for me.
Like I said, crazy thought.