Answer to Question 1
If serum B12 levels are measured, they do not reveal the amount of B12 on a tissue level. Serum B12 could be normal with a low tissue level. Methylmalonic, a metabolite in the B12 pathway is a more accurate indicator of B12 on the tissue level.
Answer to Question 2
This should be done in a schematic format and should include the pathways involving homocysteine conversion and illustrating where the B vitamins are involved. These pathways are available in basic biochemistry texts and most advanced nutrition texts.