We know the girls were on the SE end of the bridge, and given the timeline, I'm confident that they were dropped off at the North end of the park. I don't believe that he would take a chance of being seen or meeting someone by forcing them to walk back to the North end of the High Bridge, but instead, as you say, he told them to go down the hill on the South end of the bridge.
Their bodies were found on the North side of the river, so it makes sense that they crossed the river at some point. I think it's very likely that the suspect murdered the girls near his vehicle. It makes complete sense that he parked at the cemetery after seeing the girls being dropped off, and saw the girls on the bridge. It might have been easiest to cross the river and enter the bridge from the SE end, turn around, then take the girls back the way he came - just South of the cemetery.
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