Yes, I'm inline with your thoughts. The car presents a problem because we don't know if it was even connected to BG. My mind consistently runs two scenarios: car is related, or car is not.
If BG parked at the CPS lot with no real plan in place, and the murders, creek crossing, etc. occurred impulsively, than maybe he found himself standing there thinking, "Crap, now what?"
Riley has said BG had to know where to cross the creek, but that doesn't definitively mean he purposefully led the girls across. Maybe he crossed elsewhere on his way out. skibaboo's map shows police road blocks from the days following the murder. W 300 N rd was blocked just north of the CPS lot, and the N 625 W (the road that runs south from the bridge) was blocked at the W 200 N intersection. That was likely due to the ongoing investigation, but maybe there's a reason they wanted to keep people out of that south end. Maybe they thought BG recrossed the creek and took the woods southeasterly, circling back north near the hwy to get back to his car? He'd be out of sight nearly the entire way. I don't know how likely that is, but I'm throwing it out there. They did ask about any hitchhikers early on.
Personally, I believe the car is connected to BG. DC might sound confused, but I don't think he is. I think BG parked at CPS lot and followed the girls on the trail starting at that 501 trail sign DC talks about, and LE knows it. I'm hopeful LE has a full picture.