Yes, impaired glucose tolerance precedes diabetes, and that is when PN can start:

http://neurotalk.psychcentral.com/thread158275.html

Carcinoid is a slow growing tumor of the GI tract. It periodically secretes high levels of serotonin, which can affect mood, and motility --most commonly causes diarrhea.

http://www.carcinoid.org/

http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001387/

Doctors don't think of it right away... so it is up to patients to
ask for testing for it for any complex hard to diagnose symptoms, some of which you have. Not everyone has every symptom and that makes it more difficult.

A high A1C with normal fasting, suggests, high carb large meals.
So cutting back on sugars and starches and eating smaller portions may help that at this point. The A1C typically suggests high spikes after eating, which level out later, and may become actual low blood sugars. This swinging is what leads to the PN symptoms.

You might look at the gluten free diet just in case...the diarrhea is due to that. Going GF for 3 months and feeling better may be your answer. Some testing is not really accurate for some people.