In my mind, it is whittled down to three possibilities. I could be wrong.
1. Something happened that day and the driver panicked and dumped the luggage and pretended to go to the mall for the drop off. The things that point in that direction are just a few. Sadly, the stepdad scenario happens a lot so history demands I look at this possibility. And she did not seem to text or call anyone after she was supposedly dropped off at the mall. That seems very odd and unlike a 19 yr old to me. And if she had been dropped off then I believe she would have been seen on a camera somewhere. And I don't believe she has been, ASFAIK. So that seems odd. And I still don't really understand the reason she would ask to go shopping. She didn't really have the money to afford shopping. { if this is what happened then LE already knows and is trying to get the evidence together, imo]
So the other possibility might explain why she said she wanted to go to the mall. Maybe she was meeting some one there. That person may have agreed to take her to the airport after they had lunch or something. So there are two scenarios here too. Maybe she agreed to meet this friend and this person kidnapped her. And sent the texts?
OR, and this might be the most likely, perhaps. Maybe she ran off with someone to get away from her old life and the stress and pressure from everything. And this person is hiding her out and they don't know how to get out of the mess thay made by staging this whole thing. This si the one I am hoping for.