I would not open up a dog with suspected MVD for a biopsy alone.
It may be a good thing to do if the dog is being spayed (varying opinions).
Of course that isn't the case here.
Urinalysis is a good idea.
You could go to the Protein C test or ultrasound.
While Dr. Center's protocol is BAT, then Protein C then scintography if that is elevated, the assumption then is all Yorkies who have high bile acids and normal Protein C have MVD I guess. That obviously can't be right but I'm not sure how often that is the case and I'm not sure when she does recommend other testing. If you want to see the size of the liver and look at everything going on, the u/s is fine. If you are looking for a shunt diagnoses, it is as low as 60% accurate for that. We did a u/s and all we got out of it was a pancreatitis diagnosis. We did a biopsy and all we got out of that was mild bile stasis.
Remember if you get everything under control and she is "completely" asymptomatic, you can start increasing her protein as long as that protein is the right type. There is really no reason to leave an asymptomatic dog on 20 or so % in my opinion. That doesn't mean you would go to 40-50% either though. Too little protein isn't a good thing. While she is having symptoms and since it may be her liver, I would drop the protein % and get her on a better type as soon as possible.
I typically look at the post number. Haven't read too much on the pre but 95% of dogs with shunts have elevated pre. Most shunt dogs have a post over 100 though.