Apologies if this has already been mentioned (haven't read the entire thread), but I think there was something in one of those emails between Casey's grandma and the grandma's friend. I remember something the friend wrote about (paraphrasing here) there is some condition that runs in George's family and Casey has it too. That might be some reference to one of them acting like they're bipolar... or something like that?
George never killed anyone and IMO he is blameless in this whole thing, but clearly both George and Cindy had their own problems. But they were functioning, well, more or less. Of course everyone has problems; the difference is, this family's are out there for all to see.
I am completely ready to believe that Casey took drugs at some time, but I just don't think she actually had any particular drug habit. One thing that led me to believe she was not into drugs was the fact that somewhere here people were discussing Cindy's meds, and it was mentioned that Cindy had a bottle of painkillers (narcotics) that she'd gotten from a dentist, and the bottle had been around in the medicine cabinet for a while. Well, that's not unusual, because I've had a family member come back after dental work having filled a prescription for Lortab and then only had to take one out of the whole prescription. So the rest of the pills were just left to sit around, till I trashed them. There are some people who, if there were any unused Lortabs in any medicine cabinet in the house, they'd be on them like white on rice. So it seems to me if Casey'd been in the habit of taking anything like that, she'd have long ago stolen that bottle.
When both Annie and Michelle said Casey wanted to have herself put in a mental ward, I could see where that should be taken seriously... BUT in fact Annie did call her later to check on her, and whaddya know, Casey said that everything was "fine" after she talked to her mother. Sound to me like she was overdramatizing. Her mother had probably given her some sort of ultimatum about something and the "breakdown" was gonna be Casey's countermove to the ultimatum, and then the ultimatum went away, so the "breakdown" went away, too.
It's almost like Casey's moods were controlled by her mother... without either her, or her mother, wanting it that way. Just another way in which this college-age woman seems to have had the emotional level of a small child or infant.