The normal range of Vitamin B12 is from160-950pg/ml. Reason for such wide range is due to Vitamin B12 is in our tissues plus in the blood so it is difficult to have a true reading. Some people have low vitamin B12 levels due to folate deficiency. You did not mention your folate levels. There are no adverse effects associated with Vitamin B12 intake from food and/ or supplementation. If you are really concerned then Vitamin B12 can be easily consumed from our diet. One egg a day (yes, it does have cholesterol, however, it is the good cholesterol our body needs), ½ chicken breast, and 1 cup low fat milk, to name a few vitamin B12 sources would help increase the vitamin.
You have a vitamin B12 deficiency, and your neurologist seriously needs continuing education courses in cobalamin deficiency. Elevated MMA and Hcy also indicate B12 deficiency. A serum B12 of 300 is in the low-normal range, but this is because the "normal" range for serum B12 is too low. You are symptomatic and need treatment. The book, "Could It Be B12? An Epidemic of Misdiagnoses," Quill Driver Books---will educate you and your doctor(s).