You bring up good points, and I will take on your challenge.
I have never believed the girls crossed the creek.
It's my belief the perp planned these murders way in advance. In fact, I think he planned everything down to the very last detail, including the route to where their bodies were found.
He was not about to leave something that critically important to the elements of running water. The girls could have slipped on wet rocks, or even managed to get away from him downstream. They could have been seen by casual observers from the bridge or other vantage points.
He wasn't going to risk that!
More importantly, HE could have slipped on wet rocks, or dropped any of his weapons, concealed beneath his jacket. He had waited too long for this day. He was not about to risk his plans to the unpredictable elements of the water.
I agree with DeDee. I don't think Sgt. Holeman was saying they DID cross the creek. On the contrary, I think he was actually pointing out all the obstacles as to why they didn't.
I'm a person who often reads between the lines. For whatever reason, I don't think he minded if this was not spelled out clearly. That's my impression anyway.
I realize people interpret his words differently, and they may be correct.
You asked how I think he got them to the other side? The exact way you think did not happen - by automobile.
I believe he had his car parked on that private road located off the south end of the bridge. Though he knew it posed a slight chance of being seen, I believe he had checked out the nearby homes ahead of time, and saw no imminent risk. He knew his car wouldn't be there very long. This crime would be quick.
It was carried out with military precision. I believe the 'down the hill,' led straight to his car where he quickly hustled the girls into the trunk. Just mere minutes after leaving the bridge.
Imo, that's why no one saw or heard them after he grabbed them.
I think he did drive them around to the other side, hence the 'phone pinging around town' statement made by Libby's father. I know various towers can have the same phone pinging while the phone is not moving, but I don't believe that's what happened here. I think the phone was actually in transit.
I don't have any article or media source to back this up. It's my opinion only, but it fits my theory of how carefully planned this crime was. The perp made sure he covered all his bases ahead of time.
So the girls crossing the water makes absolutely no sense to me, from the perp's point of view.
Again, I may be completely wrong. There may actually be 'nothing' to even be read between the lines. Wouldn't be the first time I've been wrong, for sure.
You asked though for the 'why and how' theory from anyone who believed differently, so this is my opinion. Based solely upon my gut feeling only.
Want to add too, I'm not at all convinced it's just one perp either. I thought it was for a long time, but now I'm not so sure...