So she crossed the bridge after 3:49PM. Sundown must be around 6PM? It would be closer to 5PM when she returned to her car? She did this by herself without any concern for the bridge condition?
Do people go there to cross the bridge, turn around and cross it again?
If Libby and Abigail arrived at 1PM and 1h7m later they had still not crossed the bridge, that suggests that crossing the bridge takes them from 1:20 to 2:07 to partially cross the bridge ... 47-53 minutes and they're not yet across once.
Let's assume a minimum of one hour each way on the bridge plus 15 minutes each way to the pick up spot. They were very tight on time. Why didn't someone who was at the park at 3:49, and who crossed the bridge one way at 5:05 not see the suspect either behind or in front of her?
I think we can assume that the suspect did not cross the bridge at dusk or in the dark, but he would have been with the girls, or walking to his vehicle, by 5 in order to have the light.
In one video, a lady crosses the bridge in 6 minutes. Actually she starts filming after they've crossed the water. So I'd estimate 15-20 minutes to fully cross. And, yes, I believe people go out there just to cross and come back.