If the jury buys she threw away her phone and urinated in her jeans and left them in a bag not far from her house as a precursor to running away, the world has big problems. But we've seen some crazy judgments before so it's a worry. The reality is Sierra had had somewhat of a rough adolescence compared to many of her peers, and it will be a crying shame if they can leverage that to let her murderer go free.
I think if Sierra and the accused had contact prior to her disappearance it would be easily proven by now and the defense would be all over that for the DNA in the car excuses. The prosecution would probably use it as well, to prove that he could have got her in the car voluntarily. There would be evidence of texts, social media messages or calls if she bought drugs off him herself, or her friends would have known about him.
I hate to see the police dragged through the mud with regards to the evidence processing. 20-30 years ago there is no way this guy got caught until he had killed a lot more young women. Now it seems that even if you get the info that leads you to the killer, unless each step of the way was processed like you were doing cancer drug trials for the FDA to approve they are free to attack it.