I would ask also at the Pernicious Anaemia Society website, there are some very savvy people there who could point you to further research. I am finding out new things almost on a daily basis about folate and B12, and coming to some conclusion on causes and effects, but it is early days yet, in looking at this stuff. People with the mutations etc are as far as I can see monitoring their own progress and adjusting doses based on their own symptomatology, and it may just be that there are no fixed and empirical answers to exactly what supplementation is needed. Theres a long way to go with the research.

MTHFR sure seems to be important to PD prevention and reversal:

http://www.ncbi.nlm.nih.gov/pubmed/23916622