--even neurologists, are aware of the panopoly of tests that can be brought to bear on neurological issues.

You could have an autoimmune neuropathy, for example. Have they tested for any of the numerous antibodies to peripheral nerve?

Take a look at the spreadsheets at www.lizajane.org. A number of us here worked on these (though Liza Jane deserves the lion's share of the credit) to make them as comprehensive as possible as regards testing for neurological conditions. Many have found these very useful to track their tests and to use as a gentle prod to doctors to get them thinking in directions they otherwise might not.