So you're basing that on what the parents said to LE?
IIRC, the neighbors haven't seen him. I don't think it's so obvious that he was at the house and if he was, for how long. The entire thing with the swamp and the Mustang could be staged. Would be interesting to know if dogs ever picked up a trail and if they did, why the search was abandoned.
I don't think he ever went to the swamp. Great tactic though, truly - these people read like serious schemers.
There are so many possibilities (Dad met Brian partway, on the road, drove the rest of the way home, etc). No one knows his route, if he turned off the phones, it would be very hard to reconstruct it. He had to get gas, but without the route, very challenging to find the footage. He had $200 cash with him and I predict that came from the ATM at Colter Bay - which is likely his last use of an ATM (I wonder if it was Gabby's card?)
If Dad did meet him somewhere along the route, he could also re-supply him with cash. In many ways, if BL did decide to flee the US, Canada would be his best chance, if he could get over the border (I have no idea how easy it is to get into Canada from Montana, but it's a very long border there). He'd blend in better in Canada and could find his way to Alaska...