OK...
So my main question is, where was Abby going? (if indeed she was walking away from school yet not towards her home?)
If I understand it right (and I've read EVERY post but much of it has been skim-reading), she turned off on her walk "home" and headed in a different direction from what we would expect had she intended to go directly home.
So, I believe Abby was going somewhere for a bit inbetween school ending and back to practise (running). She had a few hours, right? Has it ever been confirmed that she was due to show up for track practise?
I think that if she was going to meet a friend from her school, she'd have walked WITH them.
So I believe that
a) she was going to meet somebody who didn't go to her school (but if that was the case, there would more than likely be social media/texting records of this)
b) she was going somewhere alone (e.g. to get some fast food, to a library, to browse the mall/shops) <-- depends on what is in the area
I'm just surprised, I guess,that nobody has come forward and said "Oh, Abby planned to do X, Y, Z after school". Friends talk to each other!
Mind you, doesn't mean LE don't know that information.