That's a good progression of your lab results. AST levels above ALT and GGT is usually associated with an area of involvement outside the liver (i.e. - heart, gall bladder, common bile duct, and pancreas).
I'm not sure that pH therapy would be the best way to address this. It sounds like you're already on the right path. Diarrhea caused by vitamin C is not necessarily an indicator of how much C you need. It generally reflects how much ascorbic acid your intestines can tolerate before trying to flush it out the irritation created by excess amounts of an acid.