I thought I would summarise the role of B12 and folate in the conversion of homocysteine to methionine.
As you can see in the diagram, methylcobalamin plays no special role. It, like other forms of B12, is converted to cob(II)alamin as soon as it enters the cell. For this reaction the cob(II)alamin is further reduced to cob(I)alamin. It is this that binds with the enzyme =,
A molecule of methylfolate and a molecule of homocysteine also bind with the enzyme. A methyl group is passed from the methylfolate to the cob(I)alamin which then passes it on to the homocysteine, converting it into methionine.
So methylcobalamin is formed, but fleetingly.
It should also be clear that taking methylcobalamin if you have a faulty MTHFR enzyme would be pointless.