--and has she had any imaging of the cervical spine and or brachial plexus/shoulder area? It is possible for compression there to produce symptoms lower down, and EMG/NCV tests may be unreliable, depending on how carefully and under what conditions they were done.

There might be a "double crush" situation here--nerves already compromised by impaired glucose tolerance, thyroid, etc. are further compromised by compression with the result that the symptoms are more than the sum of the component parts.