I created a mod where I replaced various alchemy effects on ingredients with frost/fire/shock damage and other effects. However, despite these effects checking only the fire/frost/shock resistance in the magic effect window, draugr (and I imagine) other undead are immune to their effects. no doubt a result of them being tagged with MagicAlchHarmful which i need to keep for perks etc. I am unsure where and if I can change the basic attributes of the undead class to maintain their immunity to poison damage but to cause these effects to register with their appropriate resistances. I don't really mind if draugr can be affected by other poisons, I just want to give them immunity to poison damage and make them vulnerable to the elemental alchemy effects.

Go find the draugur/skeleton races. They probably have an ability attached to them which gives them resistance. Go edit that ability under spells and modify the values there.