Drugs are one of the major causes of myositis. So I'd look to your medications carefully. People can have idiosyncratic reactions to any drug. So don't discount that possibility...often it takes a decade or more for a drug to get on any list as causing problems, once it is on the market.

Autoimmune disease and infection, are also possibilities.

Many organic solvents cause damage to the bone marrow.
Did you get tested for bone marrow peptides/antibody excess?
Gammonopathies?
Exposure to some you've had but you may have been exposed to others you are not listing here.

titanium tetrachloride:
http://www.epa.gov/ttn/atw/hlthef/titanium.html

anhydrous ammonia:
http://www.ndhealth.gov/epr/resources/anhydrous.htm

The allowed low for B12 is outdated by 10 yrs now. The new low is 400pg/ml. I'd get your levels up to 1000 or more now.
How did you get so low? Do you use acid blocking drugs?
The reason for your being low and how long you were at that level, would impact your whole body. Are you vegan? If you were low for a long time, it will take a while to correct that.