--of course, is evident plaquing on MRI of the brain or spinal cord consistent with MS lesions.

There's still controversy in the MS community as to whether one transient neuropathic episode of at least 24 hours duration with such MRI evidence is enough to diagnose MS; the national medical associations still lean towards waiting for there to be a second such episode separated from the first in time. (And not all MS patients start with this "relapsing-remitting" pattern.) A number of other conditions can cause a one-time neurologic deficit in the brain or spinal cord that might mimic MS, such as ADEM (acute disseminated encephalomyelitis). Still, symptomatically, it's often difficult to distinguish MS from problems that are more peripheral, the symptoms produced from central nervous system lesions can exactly mimic those of peripheral nervous system problems.

Looks like at least MRI's of brain and spinal cord with contrast are in order. My speculation, though, is that you're more likely to have a peripheral problem after all this time with constant and slowly progressive symptoms, which is much more a peripheral neuropathy pattern.