Have anyone thought about putting up a billboard with her information right in the the area of the hwy where ahe was last seen? So many people drive through there it is quite possible someone might remember her or picking her up.
Did the investigators happened to get a sample of what was being smoked? It very well could have been laced or mixed with something to cause a bad trip or hallucinations. The video her mom shot wpuld be of importance to investigators on her state of mind and if she was possibly having any psychosis.
Have shelters and homeless camps been checked in surrounding areas?
Was she at all suicidal previous to this event that might have triggered her?
Were her school mates asked about places they liked to go and hang/hide out?
Did they check on who she followed on social media?