No, IMO I highly doubt Brian would walk because the scenario you described likely would not meet the threshold of "reasonable doubt" (although ultimately that decision would fall on the jury). I think the biggest roadblock to the hypothetical scenario you described being enough to create reasonable doubt is the fact that the location where she was killed was not (from what I can tell) a well-trafficked or even moderately-trafficked area. Spread Creek is a dispersed campground that, from what I can tell, is quite out-of-the-way and seems to be visited only by other campers rather than people just passing by. In the Red White & Bethune video that presumably was taken around the time of Gabby's death, there are only a few other campers and they are quite spread out.
This, IMO, will be a problem for the defense because it requires convincing the jury that someone happened to stumble upon Gabby at this remote dispersed campground. MOO the location alone strongly discounts the killed-by-someone-passing-by theory and would probably be even further discredited by the prosecution.
Since it seems unlikely that someone who did not camp at Spread Creek would drive down the road past the spot where the van was parked, a slightly more believable theory might be that she was killed by another camper. That alternative quickly slips out of reasonable doubt territory too, though. We know that the FBI has asked anyone who used the Spread Creek between August 27 and 30 and may have seen Gabby or the van to come forward, so my guess is that most people who were there will have been interviewed by authorities and thus effectively ruled out as viable 'alternate suspects'. MOO.
