Amino acids are not going to provide B12 for you. Protein in food, from animal sources is broken up by acid in the stomach and about 2-4mcg of B12 may be released then and if you have intrinsic factor working (pernicious anemia has no intrinsic factor),
then you may absorb some from food in the small intestine.

Can you take a photo and post it here of your "rash"? Is it just redness with no bumps? Since it comes and goes, it implies a circulation issue or an allergy to your socks or something similar.
Sitting for long periods of time can allow blood to stagnate in the feet and lower legs, and this clears up quickly if the feet are elevated or you walk around a bit. Do your socks leave indentation marks on your ankles when you remove them? This may impair circulation further. When circulation is restored to a body part, there can be a feeling of warmth and tingling for a short time.