--that B12 level of 496, pre-supplement, is too low, despite what the reference range is.

Many nations that are not the United States use a reference range that begins at 500 or 550 and hos no upper limit.

Optimally, a serum B12 level should probably be over 700; many of us like to keep ours around four figures. B12 has no known toxicity level.

This does imply that at least some of your neurological symptoms may be related to malabsorption.