Why would dad have thought Gabby was at the Laundrie home, if he didn't know that BL had arrived there?
Naturally, the parents are thinking at first that both Gabby and BL might be missing. Gabby's mom in particular knew something of their schedule and was thinking by now, they were outside the Black Hole of Yellowstone's "no service." Gabby was in regular contact, but then Mom got the suspicious email (she might even have thought Brian wrote it - but. didn't think a thing about that. at the time).
Gabby's mom doesn't try to report her missing until Sept 10. She doesn't know the van is back at the Laundrie house - who. would immediately jump. to "Brian killed our daughter"?!?
The fact that the Laundries didn't respond to text or message was explained, at least short term, by their camping - it must have been known that the family. had made several camping trips to the same place, might be off grid once in a while. Everyone gets to go off grid once in a while.
Then...it becomes known that the van is at the house.
But you're saying that after Sept 10, you would have personally driven to the Laundrie's house? Why wouldn't you have driven to Wyoming to search in the last known place? That would have been my first thought.
I would not immediately suspect the Laundries and can't wrap my head around the fact that the Laundries turned out to be people who didn't answer texts or phone calls after Sept 1. But Gabby's family didn't know the van had been there that long...