Why Did Marlene Exclude Courtney's Character?

I've had a theory for a very long time that she didn't because her version of Alison was Courtney meaning she combined two Main Characters into one.

Biggest Example I can think off of the Top of my head is the reveal of what happened to Alison that night. But instead of dying she survived. That's why Alison character personality shifted because she was never actually just Alison, but Alison and Courtney.

Thoughts?