This is purely speculation on my part. What if this area was visited on the other recent Castle Rock visit. What if the purpose in that trip was to work out the plan. Where to leave the van, where to leave evidence in the direction of the current. But the problem is it didn't fool anybody that the items just washed there. What does she want found? The half empty wine bottle for sure, one of her shoes ( which she had already removed in anticipation when visiting the Williams), a man's shoe, items belonging to her son like his inhaler and the balls. But, she overdid it. She lumped the items too close together too far above the highwater mark that tipped investigators that they did not simply wash out of the van to the spot. This may have been to buy insurance just in case investigators did not believe they simply washed away and drowned when the van got stuck. The man's shoe, if it doesn't belong to RS, then it could have been bought at a thrift store to throw in a possibility that they had been abducted. But, that doesn't work for real. I don't think there were ANY strangers involved unknown to Shantina. I think she knew whoever met her that night.
I am not sure if the meth/crack pipe(s) found in the glovebox are real evidence that she was into those drugs or purchased as a prop, like the wine, to further sway any investigators that she was under the influence at the time. But, I would speculate that she was stone cold sober throughout. I think she planned to get rid of Az because of the influence of someone, not something. I think she planned her disappearance. If this sounds elaborate, then think of the consequences to getting caught. Every time I read something more about this case, it is the only conclusion I can draw from the facts.
I think the reason she was barefoot and crying somewhat is because she had already been to the spot before meeting with whoever. I think she wanted one last call to her grandparents and was upset about what she was about to do. She should have changed her mind and kept on going. But she didn't. I hope the investigators watched the whole tape to see if she had been down that road earlier than her first visit and her second bypass at around 10:10.