Very thorough & just the fact that you included the “missing” 23 mins has really got my attention.
IIRC BP & MP have both stated the girls were at the trails/bridge taking photos. We must consider that they were stopping along the way, Libby was taking photos & the girls were talking. Let’s assume they were also checking Libby’s phone, maybe texting friends, checking SM, looking at memes, etc. We need to also consider that they are not walking at a brisk pace. They are leisurely walking toward the bridge. This is when I believe BG either saw them or he could hear them. I think he hid out of view & once they got closer, he came out of the woods...that in itself would be creepy to me...maybe it startled the girls seeing him coming out of the woods? (I believe he was already at those trails prior to the girls arriving.) He walks by them & waits until the girls are at the other end. The girls took their time crossing the bridge as Abby is trying to be cautious due to the condition of the bridge. Libby, I think, is probably use to it & has no issues crossing at a decent pace, so she is walking ahead of Abby, taking photos. The girls reach the end & BG is already coming towards them...he makes his move...
IDK where the heck he parked his vehicle, IDK where he exited the trails but I am 100% certain he was at those trails prior to the girls, waiting for the most vulnerable of victims...
Not as thorough as your well thought out post, but I hope it makes sense. All just my own thoughts, right or wrong. MOO