--this sounds like a chemo-induced, or at least chemo-enhanced, neuropathy (you certainly could have had the beginnings of small-fiber neuropathy from diabetes/glucose dysregulation and your chemo regimen exacerbated it).

Many oncologists shy away from discussing neuropathy as a side effect of chemo and radiation treatments; it's our of their comfort zones and they don't know how to deal with it, though it IS one of the most common side effects of these treatments. Chemotherapy agents, particularly the ones based on metals, are notoriously neurotoxic and, as Mrs. D notes, when one is about to undergo these it makes good sense to try to get one's nutritional status enhanced before these agents are introduced.

Still, you should be taking more B12, even with those readings--many of us want to get our B12 readings at the very top of the range or even above, so that there is always enough available for repair and to support enzymatic functions, which these agents deplete terribly. (B12 has no known toxicity level, and many of us use methylcobalamin forms to avoid the need to have to methylate the commercially produced cyanocobalmin--there are people who have genetic errors in methylation processes, and chemo tends to throw that chemical pathway into conniptions anyway; besides, why force the body to get rid of cyanide?)

And yes, diabetics need to watch their magnesium and Vitamin D levels more than most.

It is possible you will get some healing over time, though likely very slowly. Among neuropathies, chemo-induced is one in which some recovery is more the norm, given the proper conditions (and a lack of a need to go through chemo again, of course, which we all hope for you). The diabetic neuropathies tend to be more recalcitrant, but one can improve there, too, with good blood sugar control and good nutrition and exercise to tolerance.
You may well be "co-morbid" in this--but small, unmyelinated fibers can heal and you can grow more of them if the things compromising them are removed.

By the way, the immunofixation tests were to detect monocolonal antibodies (M-proteins), which not only can cause neuropathy through interacting with nerve epitopes but are often associated with blood cancers. So good for you that the patterns did not indicate any.