Not sure about the SM issue.
But it's possible she was moved from one location to another.
It's also possible she could be on private property, not necessarily to the knowledge of the owner, but more because it's not a place LE would necessarily look, such as a remote area, or a vast area. She could be in another county or even state. Wherever she is, I believe the responsible person(s) was calculating, as opposed to dumping quickly or just finding what seemed like a remote place.
I don't believe she would be in the PTL area (which seems to be the obvious case) or even near the actual crime scene. There may be multiple scenes depending on how this unfolded and the concerns of the responsible person(s) in obscuring the crime and its evidence, and trying to ensure she's not found. I think the failure to find Lauren Spierer is due to these kinds of fears because of the greater story that would be told by the condition in which Lauren would be found.
What happened to HE may tell too much of a story about who and why, and the hard to find location is related to that.
Just my thoughts.
I believe you got it exactly right about the location. I think the perp(s) had a pre-planned location to dispose of her well before they did whatever they did to her. I think they had from 3:40 am to 6:00 am to carry out their plan. I wouldn't be surprised if she was an hour or so away from Myrtle Beach.