If you are dealing with liver disease, it's the type of protein, not the percentage that is the problem. Animal proteins from red meat produce ammonia which an unhealthy liver cannot process properly.
Liver | B-Naturals.Com Newsletter
My Lady has liver disease from years of phenobarbital from seizures. I homecook for her now, a special recipe for liver compromised dogs. The protein is about 24%, but the protein is from whitefish, cottage cheese and eggs.
I would not put a dog, especially a puppy, on a low protein diet without consulting with your vet. Too little protein can actually damage the liver.
You have to calculate protein based upon dry matter to get an accurate reading, not just what is written on the bag of kibble.
Dry Matter Basis… The Only Reliable Way to Compare Dog Foods
I personally would not try to adjust diet on my own without consulting my vet after further testing is done. You may want to consult an internal specialist if your regular vet is stumped. IMO, you need a diagnosis first, then modify her diet if necessary.