I'm so sorry your little baby is sick, and with no answers coming...very scary to go through
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You are very smart to be thinking about going to Texas A&M or similar!!! I would do that asap.
There is much more you can do for diagnosis. The following is from a paper by Dr. Karen Tobias, a preeminent Vet in shunts:
Diagnosis of Portosystemic Shunts
Increased bile acids or ammonia or presence of ammonium urate crystals in a puppy with clinical signs of hepatic encephalopathy is suggestive of portosystemic shunting. On plain abdominal radiographs, a small liver may be seen. Animals with shunts may also have large kidneys or calculi in the bladder or kidneys (urate stones are usually radiolucent but will be radio-opaque if they also contain struvite). Shunts may be detected during ultrasound examination of the abdomen; intrahepatic shunts are easier to find with ultrasound because they are very large. Even if the shunt is not visualized on ultrasound, other clues may be found, including sediment in the kidneys and bladder, increased kidney size, decreased portal vein size (if there is a congenital extrahepatic shunt), and turbulent blood flow. Another method for detecting shunting is to inject agitated saline into the spleen and to simultaneously ultrasound the right atrium or distal caudal vena cava for air bubbles.
Portograms provide diagnosis and location of shunts in all animals. Radio-opaque, sterile, water soluble contrast is injected into a catheterized jejunal or splenic vein (maximum total dose, 2 ml/kg). Radiographs (left lateral is best) are taken 1-3 seconds after beginning the injection, or continuously if a fluoroscope is available. A direct injection of the spleen can also be performed; however, less contrast reaches the shunt quickly and contrast that remains in the spleen may obscure the shunt itself.
Diagnosis can also be made with nuclear scintigraphy. Trans-splenic scintigraphy (injection directly into the spleen) provides a portogram in some animals, and in about 70% of animals allows differentiation between multiple extrahepatic and single congenital PSS. An overnight stay is required because animals are radioactive after the procedure. The gold standard for shunt diagnosis is dual phase computed tomographic angiography, which allows reconstruction of the vessels and identification of vessel number, location, and termination.
Differential Diagnoses
Any liver disease can cause bile acids and ammonia to increase. Dalmatians and other breeds can have urease deficiencies that result in production of ammonium biurate crystals. Hepatic encephalopathy must be differentiated from distemper, hydrocephalus, hypoglycemia, toxicities, and epilepsy. PVH/MVD causes the same clinical signs and laboratory and biopsy changes as extrahepatic congenital portosystemic shunts; however, results of advanced imaging techniques are normal. Dogs with acquired multiple extrahepatic shunts may have ascites and coagulation abnormalities. In these dogs, coagulation should be checked before taking a liver biopsy.