I suppose it might depend on how quickly they could have gotten back to the NW end. The photo of AW was taken at 2:07pm, and LE have stated that they encountered BG at ~2:30. There were other hikers on the bridge at or around 3:00pm, who saw and heard nothing. The walk from the NW end of the bridge to the CS is several hundred yards, iirc, so I would assume that when the other hikers mounted the bridge, according to your theory, BG and the girls likely would have still been walking to the CS, or would have just arrived. If that were the case, I would also suspect the other hikers would have heard something. I suppose BG could have had them bound and gagged, but i would still assume a scream or some evidence of struggle would have been audible. Had they crossed the creek, assuming they were killed where they were found, and no SA, they would likely have been dead, and BG exiting the area ~2:45, with the area being silent and seemingly serene by the time the other hikers arrived ~3:00. I am not saying you are wrong, just that it seems more likely to have been completely over with by the 3:00pm mark, assuming they were killed where they were found.