Hello there!
First sleuth... 😉
I'm 'studying' this case too (thank you @Bob Saccamano for the map and the sharing of your thoughts & same for everybody here).
I wanted to share mine also. On Libby's last "Snapchat" around 14h07 with Abby on the right side, well I would also suggest there is someone in the background walking on a path between the trees... Someone would know if there is actually a track over there? I don't see it on maps.
If I look at this Snap, the light and shadow don't seem to me different than the ones featured on the extract of the video where we can see the POI. So to me if it is from a video then it was one taken by one of the girl (Libby) very shortly after the Snaps. I think this also because on the second "picture" of the POI at the left side of it there is like a 'thin grey line' which could be a thin piece left on the extract of the hoodie Abby is wearing that day with hands in her pockets as we can see on the Snap.
If this, the teens seem to have no knowledge of who is this man or at least his appearance as then, he would have been more opened because so near and with none around (as shown by the other Snap of Libby with only the High Bridge) but still not looking for eyes contact...?
Also the way the person carries himself and the clothes he wears, I have to say look a bit similar to the owner of the house where the Police was. Nevertheless if he really was the POI, I guess a statement would have been made about it.
Sorry, my english is not fluent! 😟