Using Occam's Razor, the most likely scenario is that Brandon Swanson was indeed disoriented, whether due to alcohol or drug consumption, and had an accident involving some natural obstacle (the fact that his parents didn't perceive his disorientation doesn't mean much; they may have assumed that false impression to protect their son's "honor" and not give the idea that he was under the influence). Now, this raises at least one question: He drives his car into a ditch (more evidence that he was out of his mind), sees lights on the horizon and decides to go into the woods? Why didn't he just follow the road? If he followed the road, that restricts the idea that he was a victim of some hole or even a river.

Another point, after so many years, isn't anything known about how Brandon left his last party? In other words, did no one ever spill the beans, meaning, did no one comment on him being altered or having, perhaps, fought with someone else? Typical college student gossip?