If you were given cyano-version of B12, you may be one of the very few who
cannot metabolize it. Those people develop an optic neuritis as a result.
(which you state you have had).

Use of activated natural B12 does not cause this problem.
Methylcobalamin is the name. You do not tell us specifics about what you took and how much.

Also many drugs cause nutrient depletions, and you must be on something for that cardiac issue?

When people have seen many doctors and NO results are forthcoming, then one needs to look at drug side effects, nutrients, and also the potential for foods to be causing this. Gluten is the most common, but other foods you may be intolerant of may cause issues too.

Optic neuritis is a serious symptom. It is trying to tell you something, IMO.

Cramping and twitching all over? This sounds like an electrolyte problem. Calcium/magnesium imbalance.