@MCEC5, Sounds like you have great medical coverage to have all the tests done that you mentioned.

I understand your hope for finding a cause, and possible cure.

I would say yes, insist on a skin(or nerve) biopsy, and further blood labs, if possible.

For me the diagnosis was "idiopathic", case closed. I tried pumping up with B-12, labs showed that to be successful, but no appreciable change in my condition.

Best of luck.