I am betting that they will find drugs in her body during the toxicology test. If the reports of the stepfather being involved in meth are true, he may have given her an overdose and killed her, panicked, got his mother’s car and drove to the neighboring county to dump the body..........Just my speculation, may not be true, but possible....
i’d speculate step-dad and his drinkin’ buddies had some ideas and things went bad.
I think that your theory is correct, more or less.