Not sure if this has been thought of before, as I haven't read every post. What if the girls took Brovan, River Forest, then Morrell to the Evansdale Nature Trail. What if their intention was just to ride on it for a few minutes, near the house, so they could be back on time. Maybe they planned to ride up to Lafayette and back to the house from there.
Now what if there was an abductor waiting in the woods off the trail for someone to come along. Maybe this person is patient and just waits for the right victim to happen along his path. The girls happen to be in the wrong place at the wrong time.
Keep in mind I am going only from online maps, so maybe things have changed or I am not seeing this right, but there appears to be an empty building on Lafayette with a parking lot that backs up to the woods along the trail. It looks like an ok spot to park and not be noticed, as there doesn't appear to be much around that area that would draw people who might be paying attention to a parked vehicle.
So now what if the abductor strongarms the girls or lures them from the trail, with their bikes, to a car he has parked up against the woods. He knows they were on the Evansdale Nature Trail, so maybe he figures that they were allowed to ride/supposed to be on that trail and decides to dump their bikes on the other end of that trail, thinking that if someone comes looking for them they'll find the bikes eventually but down by the lake and will be mislead about what happened because the abduction actually took place at the opposite end of the trail, near Lafayette.
I don't think this works with the guy watering his lawn who says he saw them, but maybe he was mistaken? And was his sighting ever confirmed? Aside from him, is this possible with the time line and everything that is known?