One consideration. Could the perp be, actually, stalking Kelsie? And just decided that the girls could do, too, because the opportunity presented itself, when he saw Kelsie dropping them off, and because Libby resembled her.
Why is it important to answer? We have discussed everyone around the girls, but if the person was obsessed with Libby’s sister, we have to look through different yearbooks, and possibly, through a different pool of POIs. And they might be somewhat older. Some might be in college, or, have graduated.
That the crime is personal, I feel. And I am still not sure that he was hunting for two girls, or maybe it was unimportant.
Also - the guy is dressed as a farmer, but he knows technology. I think the white things in his ears on the photo are earbuds. And yesterday I was told how one could put a tracer on a person’s cellphone. You don’t need to be a super hacker, but have some knowledge of where to look for the malware..
I also thought that if he were following the girls in a car (and he probably needed a car to get out of the area fast), then, he knew where to park it. So whoever he is, he used to at least to come to Delphi when he was of driving age.