Did their grandmother think it was them from the beginning and actually see them, though? I thought she'd said they left the house around 11:30-12 and then modified the time a little when the surveillance video came out. Not saying anything bad, it just might've helped her remember better what time they left. It was one of the way early articles I'm thinking of so it's probably been completely modified by now, but if I can find it I'll post it. My original understanding was that she saw them when they left and started looking an hour or so later when they hadn't returned. IIRC.
I know what you mean on the images, my first thought was how on earth could they tell it was the girls? But there might be better or enhanced images that were used to ID them. I hope.
In one of the videos (all the same video but a few different variations were released with different levels of zoom), you can also see some shadows behind the white trailer before and after the girls pass that to me looked like it might be someone walking by. I hope if they were able to enhance it enough to identify the girls they could enhance it enough to identify that person/people too, or tell if it was just shadows. If it was a person, for all I know they were taking out the trash or something but it'd be good for LE to know who it was and if they were either involved, or saw anything. IMO.