Sorry but many of those smack of something far different than B12 deficiency imho. Some of that sounds like electrolyte and Krebs cycle problems. But what you describe really smacks of hormonal problems possible secondary to immune system dysregulation. I realize it is maybe too early to pass judgment on the mb12 and methyfolate protocols but so far no benefit to your fatigue, correct? And I would definitely suggest you continue with it. But as for your symptoms ... I went through something similar to what you describe (though the low BP eludes me and was not one of my symptoms) in 2009. For me it was an autoimmune disease except in my case I had a ton of pain as well due to the specific nature of the autoimmune disease. That does not have to be your trigger but please be open to the idea that you may be facing something other than just b12 deficiency. I also do not believe a heterozygous MTHFR C677T is your main culprit ... not by a longshot. Have you had your thyroid and adrenal outputs checked? And I don't just mean TSH and cortisol btw ... On the other hand is your serum sodium and potassium levels in range? What about magnesium and potassium RBC? Those muscle cramps, shortness of breath, and heart irregularities really suggest electrolyte imbalance assuming no underlying actual CV problem. Believe it or not the fatigue may be secondary to more primary problems. Methylation support can be a useful adjunct but if the other issues are not addressed it is unlikely to solve things. Good luck and God Bless!