I have an alternative.
They meet up with like minded people at Spread Creek. He leaves her at the van or they set up the tent nearby and goes off on a hike (leaving Gabby , he thinks in safe hands. I'm going to add to I think they had probably had an argument too and its agreed to give them both space).
He gets the lift(s) back to the van - no sign of Gabby, the tent or the other people. He spends time looking in the area of the tent, but its getting dark and he cannot find her or the tent. Her phone is going to voicemail - he thinks its over and she has left him, so he decides to drive home.
Well, then they should have been seen in the actual campground in Spread Creek - where many people had cameras. Yet, the van was only seen about a mile away from the nearest neighbors. Gabby, who doesn't like outdoors, doesn't walk a mile to try and meet total strangers. They are in an illegal campsite and not near any other campers. The Bethunes would have been in that other campground as well.
So there was no way to "leave Gabby". in safe hands that were a mile away. And in the legal campground, everyone can see and hear everyone else.
Why did Brian make the turn onto that spur road, and park at a place that is not available for camping - but instead, for target practice and archery? Did either of them know they were camping illegally? They. sure did know that Dyrt.com said to camp in a designated space (or to join up with someone already on such a space).
Since there were never any other campers near Gabby, I don't think your theory works. There wasn't even a parking space for another set of campers.
Why does he not look right next to where their own (stolen) tent must have been? Because I do believe the reports that say there was evidence of them camping at Spread Creek. Did he leave Gabby without a tent, without a tarp, with nothing to sit on? Why didn't he get his flashlight and signal around and call for her? Then, wouldn't he have headed to the other campers (about a mile away) to see if she was there? Why would he just take off if he thought Gabby was socializing (and take her phone, van, and, apparently, all her things?)
Or do you think he offloaded her stuff right there, so she could find it later, took off and ravens got to it? I find that very improbable and I believe she would have been found sooner had he placed her phone and other things where she could find it. Did she have her wallet? Why isn't he willing to tell that? Why hasn't she used her own credit cards - did he take those cards out of her wallet BEFORE she went to socialize? Why?
I am for Occam's razor. He had her wallet and her phone, he had her credit cards and probably her Apple Wallet too.
How do you explain him walking to Colter Creek (he doesn't take off right away, the van is still there) and then hitchhiking back?
Why not just open his eyes and see Gabby (who would have been quite visible) by walking a hundred yards or so and flashing a light - or waiting until broad daylight?
Wouldn't a. person alert authorities if their fiancée. appeared to have been kidnapped?