Low Mg, Ca or K can all cause QT prolongation
Vitamin B12 needs to be transported to be absorbed. Some people don’t transport well. That’s why we supplement over RDA (saturation also increases absorption).
Cyanocobalamine works better, especially in older adults. You don’t acidity to absorb it. You can give 1000 mcg twice a week, 2500 mcg once a week or 25-100 mcg daily.