Would i harm my body if i'd have a mthfr defect and supplement with normal b12 ?

Probably not: Regardless of whether you have an mthfr mutation in both genes or not, the treatment for elevated homocysteine is the same—dietary intervention and supplementation with Folic Acid and vitamins b6 and b12. If you have mutations in both mthfr genes but have normal homocysteine levels, you do not need to be on Folic Acid or vitamin b6 or B12 therapy.

Depends: What do you mean by 'normal' B12? There are 4 different forms of B12 available on the market. Cyanocobalamin is most commonly available, and is the worst form. Methylcobalamin is active B12, and should be considered the normal form. You should take methylcobalamin in usual doses. (do not mega dose without a reason).