I have to recommend that you take your dog to a specialist at this point.
Internal shunts are most often found in large breed dogs, it's highly unlikely to find an internal shunt in a Yorkie... as Liver Shunt dogs have difficulty with anesthesia, I would only approve surgery for shunt repair... and an experienced vet that specializes in shunt repair would be the best option.
Liver Shunts are very hard to find on ultrasound, only experienced vets are able to do this ... a vet with less experience with Liver Shunts will most likely miss finding the shunt.
Your dog's BAT results are quite high. I don't think medical/dietary management would be a good choice for her... 5 years old is not that old. I know of a 7 year old that had the surgery and is doing fine.
If you can go to the University of Tennessee, they are the best in shunt surgery:
Contact
They will also discuss your dog's case with your vet if you ask him to call them for recommendations on a course of treatment for her.