My thoughts are
1) If she was meeting someone for a clandestine meeting, I would assume they would park "out of sight" just in case a camper did arrive early.
2) Now that I have seen NIN's image of the car, my second thought is that MB met someone at the church, they picked her up, they drove around for their meeting, then that person dropped her back off at church. "Car guy" either knew SP was in that church waiting for her, or, they were unaware. SP in church knew of this clandestine meeting, and knew MB would be back anytime between car ride and 4AM to set up so they just waited around for her. "Car" guy dropped her back off and left, or, parked around back and waited for SP to do the murder and took them away.
3) Camper's help to set up, they park, help, do the murder, jump in car, throw off clothes, or stash clothes somewhere, jump in car and circle around building to make it look like they came when everyone else did. If they were able to drive off before 4:35AM, then this would work. TIGHT timeframe though, I know.
I'm really leaning more towards #2 though. I've always thought it was a pre-arranged meeting but never really thought about someone actually picking her up and taking her back though. Just assumed meeting would happen at church.