I'm glad to hear you are having more investigations of possible cervical involvement. Although your B12 levels can certain cause SFN, it doesn't typically present with such a rapid inset as yours did. The onset and your accident wasn't a coincidence...it has to have something to do with it.

Autoimmune conditions are 'triggered' by most commonly infection of some sort, but they can also be triggered by trauma...in the form of surgery, injury, etc...anything that causes physical trauma to the body where there is an immune system response.

Of course, there could be some hidden damage the MRI didn't pick up (did they use contrast?).

At least they are looking to make sure nothing is going on in the neck. And regardless of what set this off, now knowing your B12 is factor and doing something about will help you in the long run.