I think that many of you are forgetting that spell damage and sneak damage are two separate types of damage they are separate and added together.
two things back my logic up Dr/Magic does not affect Sneak attack damage and Fortifaction in no way affects spell damage even on a Crit, I do not even think that Harm gets bonus damage on a Critical Hit. To deal Critical damage you cannot deal a fixed damage amount like Harm deals the spell would have to deal a variable amount of damage like Shocking Grasp
or fireball. There are two Ways that I can think of dealing fixed amount of damage on a Crit but they are both from the Mythic rules nothing to my knowledge in non mythic rules allows fixed damage on weaponed critical hits. IMO Harm was a poor example spell to Pick by the OP If he wanted a Necromatic Spell that could deal Critical damage I would have selected Vampric Touch.