I can't see any reason for skin or nerve biopsy, as far as i know it only shows how much damage there is to the nerves, it doesn't give any more clues of why this is happening, i would think it would be only usefull to use in comparing the damage over time, but thats easily done by how you feel anyway [improvement or worsening] of the disease i mean.
I had small nerve damage and just to confirm it, my neuro sent me for a test that measures how you react to heat and cold sensations and a vibration test as the small nerves job is to do excactly that, its a very simple test that is only done to evaluate the damage of the small nerves.
I hope your neuro is an expert in neuropathy because there are many that are not and will take you on a merry go round.