Deborah, thank you for the update.
Your serum amylase doesn’t tell us much about the pancreas’s ability to produce digestive enzymes. The best test for exocrine pancreatic function is still fecal elastase. Sorry, I didn’t realize how expensive that test can be in the US.
That said, your metabolic picture is quite interesting, and it still makes me wonder whether a mild subclinical exocrine pancreatic insufficiency could be playing a quiet role – even without obvious loose stools. The pancreas can remain remarkably silent for years, and even a small reduction in digestive enzymes can impair protein and fat absorption, which may be limiting your muscle protein synthesis and making gains harder than expected.
Of course, it may not be the main issue, but given your very low RMR and difficulty building muscle, I thought it was worth mentioning as one possible piece of the puzzle 
Regarding your plan to titrate off metformin toward imeglimin: that sounds very reasonable. Many people notice better exercise response and muscle gains after reducing or stopping metformin.