I had the silliest idea about Erda
Erbus didn't kill her... he changed her. Not necessarily on purpose, mind you, but he thinks the end result is pretty funny and it's pleasing to the chaos gods so no further correction is necessary. Cultist *is* Erda post Heresy after barely surviving the process of chaos corruption and losing her sanity in the process. Being, largely, an entity that exists outside of space and time it's no surprise she occasionally goes back to the neolithic era or slips into the Throneroom to occasionally visit the person who is most important to her, but her perception of reality is totally disjointed from what everyone else experiences.