Most scenarios seem to involve BL doing harm to GP, intentionally or otherwise. What if GB and BL had another fight late on the 26th which simply lead to him going off alone in an attempt to separate temporarily like the police had split them up days earlier. He left her behind in the tent and something happened to her while he was gone (something self-inflicted or maybe accidental). By the time he returns and finds her, he freaks out and takes the van straight home.
Under normal circumstances you’d think he would just report it but they both potentially seem mentally unstable judging by the evidence. What if BL’s real crime here is what he did with the body after the fact, due to panicking, rather than what he did to her?
One of the things that lead me in this direction is his discussion with the woman from TikTok. If you’d killed someone, would you even mention them to a stranger let alone talk about them being back at the van, etc? What if he cleaned himself up to return to her and it was too late?