The more photos I see of the bridge and the shape it's in and the area, the more I think something like that could be possible and I was just thinking that myself this morning. I know as a young teenager of a couple times a friend and I saw a "creeper guy" or someone that raised our fear from following us or who kept appearing where we were, we tried to just get away from them and that included being in a wooded park once and going off trail and just hauling booty through the trees in a straight line, once we lost line of sight with them on a curvy trail.
I was also thinking, growing up in a small town as a teen, it wasn't unusual for us to just wander around parks or areas like the one they were in. Or just go out for no reason but to hangout. I kind of don't even think there was a specific reason they were out there, other than to just go somewhere that kids their age hung out and take photos, etc. I know kids now have more technology and such, but I feel like it's probably not much different than when I grew up in some ways. We used to ride bikes down the dirt roads from our houses, 2 miles into "town" to go sit at a picnic table at the park. For hours.
I don't know that I think BG/BM knew Liberty and Abby specifically were going to be there...I wonder if he just knew school was out, kids hung out there and was in general looking for anyone as a victim. I really think anyone out there that appealed to him and was there and the timing was right for no one being around, could have been the ones.
*shrug* Just my thoughts. I'm not much of a sleuther compared to you all!