Before Mark climbs on top of Peter's car to see why the traffic is stopped, you can see Peter laughing through the windshield.
Mark finally shows up at the church, comes in through a side door and goes to the priest, who nods to the organist to begin. Elizabeth and her parents did not know Mark was there, but when the music starts, Elizabeth and her father are in position and start walking down the aisle, and her mother is sitting in a pew. Before Mark showed up, the three of them had been waiting for him in another room or office.