Well, if you are feeling better on the low carb food, that is telling IMO. So keep at it.

You don't have to have had the antibiotics "recently" that I mentioned to be affected. People have been permanently damaged from fluoroquinolones and Flagyl.
You could have had them 10 yrs ago... and have PN from them.

Also exposure to solvents is problematic. That exposure could have started an assault that was joined later by other factors.

Sometimes PN is more than one trigger. Some people have several in fact...which add to each other, to damage the nerves.
Even vaccine exposures can do it.

You are 40... wait til you are 60+ . Controlling this now, will help delay later in life issues with blood sugar.

Have your neurologist read this paper:
http://www.citeulike.org/user/wrs1/article/6499468
He needs some updating.

If you use a mixture tablet for your calcium/magnesium check those ingredients carefully. Most are OXIDE and magnesium in this form is not absorbed. It is useless. A quality magnesium product that has good absorption, will help cramping tremendously.