My impression was Julie was home alone when she called her mother for permission to go with the girls. So by that reasoning, my guess is she would also have been alone when dropped off later that afternoon (before whoever else in her household got home). I'd take "be home by 6" to be about Julie being back in the house and not losing track of time at the mall, but not necessarily being with anyone at her house. It also makes sense her mother got home around 6pm from work, so she wanted Julie home when she got there.
I'm thinking now perhaps the trio got in a 2nd car at the mall thinking its driver would take Julie and Renee to their respective homes. But why? Because 2nd car driver had a female passenger, with the claim that she needed a car for her own errands either at the mall or elsewhere. So...2nd car driver takes the trio in while female passenger gets out...Rachel hands over her car keys assuming she'll see her own car and the female passenger later. I think they got in the car without fanfare looking completely normal, because it WAS normal. These people knew each other and no one was under duress...there was nothing to witness and so it wasn't. That's the basic foundation before myriad scenarios may have unfolded.
From there I think the intent WAS to bring at least Julie (and probably Renee) home, but they witnessed something. "Something" being either violence towards Rachel or they heard something said between Rachel and the driver. Or both. If it was just something said, it was incendiary enough that the driver didn't want to risk it being repeated, i.e. extraordinarily sensitive info about people/places/things. In that case, it's likely whatever was said put the driver in jeopardy and that sealed their fates. But I do still lean toward something pretty dramatic and point-of-no-return-ish to lead to the killing of a small child. So, maybe a screaming match on the way home...driver pulls over and he and Rachel go at it outside the car. Rachel goes down, and doesn't get up, perhaps she is shot. In the heat of the moment, "kill the witnesses" might be the only clear thought of the driver.
Meanwhile, the female passenger who took Rachel's car could truthfully say she didn't know what happened or where the trio ended up. In this scenario, only 5 people knew of the car switch and 3 of them are likely dead. Rachel's car may have been taken for some errands and returned later after everything hit the fan, or maybe it never moved from where she parked it. Any prints in her car would make sense. The 2nd car might have been unknowingly "borrowed" for an afternoon and returned unnoticed, or it may be underwater or buried with 3 bodies in it.