--was way too low. I'm glad that you seem to be coming up now with supplementation, but the fact that you are up to "only" 674 after supplementation (how long has it been that you are supplementing, and with what dosage?) does imply a long-term deficiency.

The B12 deficiency, if fairly long standing, might well explain most or all of those symptoms. It's also a sign to look for autoimmune conditions of the digestive tract, such as pernicious anemia or celiac; B12 malabsorption is often linked to these (especially in the young).

The chemical exposure angle certainly needs to be investigated as well; there are certainly many neurotoxic chemicals. It may well be that there is more than one thing going on. Also, and not to put too fine a point on it, given that you have been overweight, have tests for thyroid function and advanced diabetes testing (such as a glucose tolerance test) been done?