Just some thoughts and musings as I process all that I've learned about this case since Monday...
Everything seems to be revolving around the driver/friend(?), what he did or didn't know, and his level of involvement. A lot of it comes down to how much of his story do we believe. If he's involved, then we can presume some of his story is false, but a good portion, if not most, of it is probably true...
First question is was he really a former HS friend of BB or just some random guy that BB met off craigslist or social media? I tend to believe that he really is a former HS friend... for quite some time the father was backing him up so presumably he knew him or knew of him. (I suppose it could have been a complete snow job but I'm going with he was at least a former friend or acquaintance)
Next thing that's bothering me... let's assume the driver was the sole perpetrator.. why would he be hanging around and so involved after the disappearance? I'd think he'd want to distance himself from the situation as much as possible. The only reason I can think of that would (somewhat) jive with that is if the death were accidental and as others have suggested he was trying to cover up his involvement. But then... would a friend really do that? So those two things seem somewhat in conflict. The only other thing we're left with is that he's a cold-blooded killer, which to me isn't consistent with him helping the investigation so much.
Now we come to the new details revealed by the affidavit. The dirt under the hands makes no sense, unless this sheriff's affidavit came back on Jan. 3. I don't think we know the date of the affidavit, or do we? Seems to me if he were involved, anyone would have been smart enough to wash their hands?
On to the theories of the body being moved - I don't think that happened, at least not by the driver that we know of. There would be physical evidence all over the rental car. The fact that there was still camping and other gear in the car tells me that there was no effort to clean the car. This also tells me that the camping gear is not really evidence of some kind of "flight plan", unless the guy really is monumentally stupid. In any case, IMHO, most likely BB was killed in the park and not moved around. Maybe that secluded area is where he had his "meeting"?
As far as someone else being involved, I think this probably depends on whether this is an "accidental death" / cover up scenario, or a pure act of violence, pre-meditated or not. Let's assume the latter for sake of argument. In this case then I'm more inclined to believe that there was another person involved and perhaps the part of the story of the driver dropping BB off in the part to meet someone is true. If so, then the question for me really becomes is this 3rd person someone that the driver/friend also knew or a true "random hookup" as has also been theorized.
I think if there's a 3rd person known to both BB and his driver friend, this complicates things and may explain the driver's questionable behavior. Maybe he's trying to hide what he knows about the other friend so as to not implicate him.
I don't know, maybe this last idea is a stretch, but there's a lot of this that doesn't add up for me. It really comes down to motive and then how someone would act after committing such an act. I have a hard time thinking a cold-blooded killer would hang around and help out the search investigation for days.
Given the facts we know, I can't put together a solid theory that I have high confidence in. Certainly it seems the driver was probably involved somehow - it's his level of involvement that I'm having a hard time being sure of.
Thanks for listening to my ramblings...