Here is information on the Protein C test to show your vet:
Comparative Coagulation -- NYS Animal Health Diagnostic Center
No way should you do an invasive biopsy at this point!
It should like your vet isn't that familiar with updated protocol about diagnosing liver shunts. You could waste a lot of time and money dealing with him. Are you close to U of T?
Cornell is also excellent if you are in the northeast.