From what Marlene said she probably was in mind as A in season 3 and that's why they introduced her, but I definitely think that the whole Charles backstory wasn't thought up until season 5 when they started planning for Allison's return and doing the flashbacks with Mrs D, but even so there are still sooo many plotholes. I wish that they just hasn't introduced Toby's mother suicide as such a big plot, I wasn't that into it in season 4 anyway when most of Spencer's story was about helping Toby look into it, and none of the timeline tracks with Charles's age as Toby's mother died when he was a teenager, and Charles herself wasn't even a teenager in the flashback!

And totally agree on Sara. She wasn't even a great actress let's be honest, and there was nothing intriguing about her character to me compared to some of the people that they could have chosen between