Soaps have a long history of taking bad girls and transforming them into classic soap heroines. I've seen it happen with Raven Whitney, Bobbie Spencer, Karen Wolek, Donna Beck and yes, Rachel Cory. What all these characters have in common is after all their misdeeds, they all went through a period where they suffered, so the audience could see them atoning for their sins. Only after this atonement can they transform into heroine status. This storyline with Sheila will be Quinn's atonement, I believe. Sheila has always operated long term operations on her enemies (poisoning, gaslighting), her plans for Quinn should be very interesting.
It's almost as if the writers said 'how can we normalize this crazy bitch?' and the answer was 'we get a crazier bitch' with the solution being 'see if Kimberlin Brown is available!'.