Talk to a metabolic doctor. The regimen for this rare disease involves VERY high doses of INJECTED hydroxocobalamin (+ other meds if your homocystine etc are high).

Oral B12 is inadequate. This condition requires a lot more hydroxocobalamin.

To be clear, I am not a doctor. I am suggesting you find one that is connected to the other doctors who have dealt with dozens of patients with this rare disease. Most docs have never seen it.