Hello WG. Your normal MRIs are very reassuring. I would suggest you now get nerve conduction testing (EMG/NCV) of the lower extremities, if not the upper extremities as well. This will give you information about the integrity of the nerves, where any problems may be, and how severe the process is. Important diagnostic clues can be obtained in this way. In addition, if your doctors deem it appropriate, you could get a lumbar puncture to assess the spinal fluid.
It sounds neurological in origin, so I would not give up on that angle. That said, you likely have a musculoskeletal component as well (or 2 separate issues), so an ortho referral and/or physiotherapy referral would make sense. Just keep in mind that orthopedists focus mainly on the bones and joints of the body as well as those levels of the spine that have ribs (i.e. not the neck or low back, which is typically the perview of the neurosurgeons).
You're welcome. Hope all works out for the best!