What great post, Crystal.
I agree that the bile acid test resulst don't look right. They should take paired samples around a meal so there should be two numbers. The samples also need to be collected properly.
Here is a great article from the vet who developed the BAT:
PSVA and MVD Research Summary
As Crystal said, ultrasounds aren't that reliable for detecting shunts, especially in small dogs. After an abnormal BAT, the next step would be a Protein C test:
Comparative Coagulation -- NYS Animal Health Diagnostic Center
Are you close to a vet school? I would suggest you take Scout there. Most local vets don't have that much experience with liver shunts. You can waste a lot of time and money and get no real answers.