Most of us here that use B12 keep our levels at 1000 or more.
400 is the new acceptable low...so you are borderline.

Both methylB12 and methyfolate need the enzymes to be methylated from inactive food sources. Folic acid is inactive until methylated.

Many people can have this mutation, and some only have it on one chromosome, so they slip by for years until something sets them off and they need more methylation to repair damage to tissues and nerves. This is called homozygous. If the mutation comes to both chromosomes, it is called heterozygous and appears earlier in life. So some people don't show the symptoms depending on how their mutation occurs. It can get pretty complicated, but as time has passed the percentage reported in studies has risen from 10% of the population to about 40% having some form of the mutation.

Read over at MTHFR.net... you'll see. At pennies a day for both methylB12 and methylfolate, it is not burdensome to use them.
Stop them for several days before testing, and be sure to take the B12 specifically on an empty stomach as food present in the GI tract blocks absorption.

Your experience with the venom may be connected, as the body may make antibodies to it, which remain after the initial incident. They can then contain peptide sequences similar to our own tissues, which then get attacked in the future. This happens with some viral illnesses too. This is called molecular mimicry.

Some PNs are mechanical --- involving compression of nerves somewhere in the wrists, ankles and spine.

Some are chemical -- drug induced, toxins, poisons, chemo etc.

Some are immune mediated-- autoimmune

Some are metabolic/hormone induced-- diabetes, thyroid malfunctions, paraneoplasia, etc.

Some are nutrient deficiency induced-- B12, B1, B6, folate, Vit D and E etc.