Perhaps what your Mom needs is to see you not eat her food. Perhaps that will knock some sense into her? I doubt letting you help her cook would be the end of her world. She'll learn. And if she doesn't-oh we'll. Move on and find other things to talk about.
I'd go - Thanksgiving is about more than the meal...,but I'd take my own food! Get a turkey breast and cook it the night before - make your own quick versions of whatever you like and pack it and take it and eat it. Yum yum.