Sorry ... long post ...
I have a brother that I describe as a "bull in a china shop". What that means to me is that he impulsive, doesn't think things through, doesn't pay attention to detail, acts without thinking, doesn't seem to have the capacity to carefully consider all of the consequences, assumes that he has it all figured out when it seems obvious that there's no depth in his figuring ... and after he has acted, there are a multitude of unfortunate consequences. I have never associated the phrase with being clumsy.
I'm guilty of initially having the girls enter the park on the east side because I immediately assumed that they entered the trail starting at Lafayette. I was thinking last night that I still hadn't been able to reduce the facts to a theory without adding new facts.
We know they went west on Brovan and we know the bikes were at the lake, so why assume they went to the east side of the lake trail ... the simplest answer is that they went to the west side of the lake trail. I looked at sidewalks from Brovan towards the lake, as I'm assuming the girls stayed on the sidewalks if the roads were at all busy. This would have the girls riding south on River Forest Road, turning east for a block or two at Central Avenue and then south on Jones. That gets them straight to the west end of the trail.
Regarding the timeline between the camera and the cyclist, again we have a conflict (4 minutes isn't really long enough to make the trip). Suppose the cyclist is riding west and the girls are riding east, but the girls have decided that they want to get to the water for some reason - an adventure, let's say. They drop their bikes on the path, the cyclist has to swerve around them, and he continues on to the bathroom. The girls can't access the lake well enough where they dropped their bikes, so they get back on their bikes, ride to the double gate, lean their bikes against the fence and enter the shore area through the unlocked gate. Perhaps it was their intention to return the way they came (avoid the "creepy area"). Elizabeth puts her purse on the shore and they walk/explore a bit. Let's suppose someone else is in the area ... someone that is parked on Maiden Lane, perhaps someone fishing or looking for driftwood ... then they vanish.