I don't know much about glucose tolerance tests, but I do know that a good way to look for diabetes is a fasting insulin. Also, when tests are abnormal, it's a good idea to repeat them.
His glucose numbers are odd.

Does Alan get symptoms of hypoglycemia? Does he get sweaty and tired, confused, trouble thinking clearly when he hasn't eaten for a while or has had a sugar load a few hours in the past? If so, maybe these numbers relate to a problem with the mitochondria. The chemical inthe body that seems most important in this is carnitine. Carnitine carries fat into cell mitochondria to be used for energy and people get symptoms of hypoglycemia when they don't function well.

There's a whole group of disorders called mitochondrial disorders which, in their benign adult forms, cause people to get severely hypoglycemic because their body can't use fat. I don't know that this describes Alan at all, but some people get hypoglycemic because of these problems. Carnitine is generally used for all these disorders, and for people who get hypoglycemic, carnitine can make a huge difference.

Mitochondrial disorders cause neuropathy, maybe because the mitochondria are so much more important in providing energy to the ends of long axons than, say, in a small skin cell.

I know that for some reason Dr Fred has decided the next step is a lumbar puncture, and I don't totally understand this, but then again, I often don't understand what is happening with Alan. But there's a set of blood tests that might be helpful if these glucose numbers are true, and those would be called "mitochondrial" screening tests. Also, a serum carnitine level would be useful. And easier than a spinal tap.

I mean, what exactly are they looking for with a spinal tap?

Polyclonal gammopathies are generally not that important, usually just showing that the body is fighting something. A cold, sinus infection, leg infection, many things, can cause polyclonal elevations of gammaglobulins. Through years of sinus infections I had polyclonal elevation of IgM, which no one got too excited over. IgG usually means it's something that's in the past and that he's fought off. At least that's a simplified understanding. Maybe somebody else (Glenn?) can explain it better.

But fasting insulin and carnitine levels definitely are simple to do.