All those things could have happened...making Casey not ACTUALLY a murderer, but a woman more interested in protecting herself than in grieving for her daughter. If she was capable of pimping out her daughter, it certainly explains her cold attitude and behavior.
The woman is seriously messed up - narcissistic with no empathy or concern for others and a pathological liar. But with all narcissists and pathological liars...there is always a bit of the truth at the root of their lies. Thus, it could have been "accidental" if it took place after some perverted action.
Obviously Cindy has enabled her throughout her life while George just gave up.
It doesn’t explain THREE pieces of duct tape, though. One to keep her quiet...alright, it’s possible (not likely). But three pieces?! That doesn’t make sense in any way other than to smother her.