To wit, In order:

1. Absolutely--a significant fraction of non-length dependent neuropathies don't reveal a smoking gun cause when investigated (mine, for example), but one has to make sure that very thorough testing is done; a lot of doctors, even neurologists, are baffled by this entity. Particularly, the immune/toxic possibilities should be explored--paraneoplastic syndromes, nerve antibodies, heavy metals, organic industrial exposure.

2. This is sort of answered by no. 1--at least for things that are known to cause non-length dependent syndromes or neuronopathies. There are serum tests for the various antibodies, gluten, toxins, and skin biopsy patterns, if enough samples from various sites are taken, can often be helpful. I've known doctors who are confused when skin biopsy results show overall damage rather than length dependency, though--in my opinion, finding that non-length dependent pattern should prompt a further search for the types of causes listed above (if not already done). EMG/NCV studies generally are not very helpful.

3. A good question-and the answer is not necessarily. In fact, there's more and more evidence that many with gluten sensitivity may not present first with gastric symptoms. There's a whole body of work by Dr. Hadjivassiliou on neurological prsentation of gluten problems--JCC has collected a lot of this in The Gluten File (everybody should have this bookmarked--most comprehensive database on the subject yet compiled--http://sites.google.com/site/jccglutenfree/)
and even today, I found this (you need to sign up for a free subsciptionto read it, but no big deal):

http://www.gastroendonews.com/index....e%5Fid%3D16015

4. My understanding of Issacs syndrome is more limited, but my understanding is that while there may be sensory symptoms involved, the syndrome is defined by irregular activity in motor neurons that causes continuous activity in muscle fibers. It is interesting that some of the same autoimmune and toxic causes are implicated in this syndrome as in neuronopathies, though.