I don't have it, but I have many friends/colleagues who have dealt with it. It often accompanies diabetes, but it occurs with many other conditions (some diagnosed, some not). Like you, several of them have been "medical mysteries" with a mix of symptoms. As you have found, many conditions have overlapping symptoms, and often diagnosis eludes the doctors until things are really bad and symptoms are full blown.
One of my friends was one 30 prescriptions. Yep, 30. Now she's down to 4 and working on getting off the last few. She had various prescriptions being put on top of other ones - some were to treat her condition, and others were to treat the side effects of the first drugs.
Some of them had fluctuating neuropathy, others found it pretty constant. Yes, it gets worse over time, particularly if the cause is not determined or, even if determined, not addressed. It can spread but it's primarily in the extremities - but "how far up" it goes can worsen.
All of them have reduced or eliminated the neuropathy through nutritional means - boosting their immune system. This is too much to put in an email but I'd be happy to connect them with you and see if you like what you hear.
I am happy to see that you have not gone overboard in researching on line. There is SO much scary stuff out there, and it's hard to evaluate the credibility of sources. Moreover, so many "established" medical protocols have been refuted or withdrawn - studies contradict each other, and many are not blind trials (they are conducted by the people manufacturing the drugs!).
I'd be happy to chat with you more. There is hope!