I think whatever happened to the girls occurred in that house also, after they returned from the Mall.
MOO, although I think they made it to the Mall, there is no strong evidence to suggest that they did. I would love to hear theories from anyone on here who believes that they didn't get as far as the Mall. It would help put new ideas out there, that may have been overlooked.
I don't think they were abducted or got into any strange cars in the Mall. The logistics would be difficult. Imagine constantantly circling around a busy Parking Lot hoping to get the girls as they come out. You would be constantly getting blasted out of it by the other car drivers !!
Even if the girls willingly got into the car, someone would have seen something. Remember this is 1974, people don't have earpods in their ears, heck, the Walkman was still five years away. People are going to be far more alert and notice things.
I think they returned to the house at Minot without incident. However, in saying this, I am also saying that the car was 'staged'at the Parking Lot in the Mall. The time frame to do this would be tight!!
Feel free to shoot holes through my theory. I would love to hear other people's differing opinions, as I don't want to to get 'tunnel visioned' on this case.