Welcome to NeuroTalk:

It is difficult to say regarding the pre-diabetes. You'd have to have a 3 or 4 hr glucose tolerance test and A1C levels to see if there are hints of reactive low blood sugar after the 2 hr mark.
You can also have a fasting INSULIN done which if elevated suggests insulin resistance. A1C is a general marker showing over time, glucose stability. Usually it is the spikes after eating that raise it. But A1C does not demonstrate LOWS. Lows, also starve nerves.

Did you have thyroid testing? Some hypothyroid symptoms involve PN symptoms.

I don't believe tonic water is what you need for cramping pain.
The amount of quinine in it is very low.

You could add in a good magnesium supplement (not the OXIDE form which is not absorbed) and see if you are better.

Diabetics lose magnesium in the urine. And most people do not eat foods providing magnesium and they become low from dietary lack.

Also I don't think you need so much Vit C. More like 500mg a day would be enough. Taper off slowly if you discontinue this, because Vit C in high dose can cause a vitamin dependency and scurvy symptoms may result with sudden discontinuance.
Too much Vit C puts a strain on acid/base control of the blood.
Using Ester C is a bit milder and less acidic.
There have been suggestions for type II diabetics to not take high dose Vit C...but those are still not proven.

Check your R-lipoic...is the stabilized form? Also called:
NaRALA. The non stabilized form is thought to be less absorbable, so check your labels.. You will want to take this on an empty stomach as well.

I would get your B12 levels checked. and Vit D. Both can be low, and often doctors say "normal" when in fact the labs report lows as "normal" . Your B12 should be at a minimum 400 US units, and with PN symptoms you should strive for 1000.
Some drugs like acid blockers block B12 absorption and people can become low from them. Also metformin used for type II diabetes will lower B12.
This is my B12 thread with medical links:
http://neurotalk.psychcentral.com/thread85103.html

Also if you take statins to lower cholesterol, be aware these can cause nerve damage. Also some common antibiotics, Cipro, Levaquin and Avelox, and Flagyl. The PNs from Cipro etc are not thought to be reversible at this time.
There are threads in our subforum above on the subject of drug induced PN and also supplements.

Also some people get PN or GBS from vaccines. Some vaccine injuries are similar to PN.

The GLA may be helpful, but you may not need so much. If you don't have side effects from it, leave as is. But if you seem to be getting worse, I'd cut back to 1/2 that dose, and see if that is better. Also useful are fish oil, or krill oil. These are anti-inflammatory. If you do not eat salmon regularly you should either do that or take the supplement versions.