If she ran away, she either walked away from PTL after little or no sleep and was seen by no one, or she sprouted wings. Or she had an accomplice who either went missing with her and we've heard nothing about it, or has managed to endure LE's scrutiny unscathed since Heather's disappearance.
Heather's gone because someone ended her life and then dumped her remains, in my opinion. I cannot imagine a scenario where she will return to her family alive.
Missing persons and murder cases are cases of lives abruptly interrupted by other persons. I believe LE can tell the difference between the trail of an interrupted life, and the trail of a life with a new direction.