Does amniocentesis accurately detect if a foetus is affected by rubella if done at 18 weeks pregnant? is it possible to draw blood from the fetus at 18 weeks as well as an additional step to test for rubella within the fetus

Doubt the need: Amniocentesis has inherent risk for mother & baby & a selective blood draw would be more dangerous. You both share the same serum markers & blood evidence of IgM antibodies from mother's blood would verify new rubella infection in both mom & fetus. A high resolution ultrasound could also pick up some fetal damage/defects.