ALT is weird...can be elevated and then come down for no explained reason. As to a liver shunt, you can actually have an older pup with a shunt that has never had elevated enzymes...happened to my Teddy Bear. I will post his link at the end of this post in case you might want to read it. They also don't always have typical symptoms...it is all dependent on the size of the shunt and how much blood flow actually goes through the liver to be filtered.
I would do the BATs with your vet as it is a simple, inexpensive test and then go from there. I personally would not do a liver biopsy for just an elevated ALT. That is an invasive test. I would go to an internal med specialist if I was concerned. Not sure where you are in Texas but there is a great hospital here in the Houston area. There are others in other parts of the state as well. You can find an internal med specialist at:
www.acvim.org I am happy to tell you about the local one if you are in this area.
As for gall bladder sludge...super common and I have never seen a problem because of it.
Here is Teddy's link:
http://www.yorkietalk.com/forums/sic...ver-shunt.html