Hopefully, some physical evidence is there. Yes, there have been murder convictions on pure circumstantial evidence, but it’s riskier. What’s considered reasonable doubt is not something measurable.
The Laci Peterson case is an example of circumstantial evidence carrying the conviction. Was it possible that her body could have been dumped where her husband happened to have spent the day she disappeared just to incrimate him, since it was found after news came out where he had been? Yes, it COULD HAVE, but with all the reason he had to be rid of her and lack of anyone else having any motive, and the difficulty of stashing her body for a time and just dumping it there did not seem reasonable to the jurors. It was not as outrageous as a Big Foot entering the scene but considered unreasonable.
With BL who does frequently go off on long walk abouts alone in the wild, is it possible he went off on one after, say a heated argument to just get away, cool off, and return to a campsite without Gabby, and he comes across her body and freaks out, panics, knowing well that the relationship was gong down hill, the DV being on record with him even so saying. Gabby possibly telling friends and family. So he bolts. Takes the van , heads home and just won’t talk about it. Maybe hopes Everyone will think she left the campsite and disappeared as some young women have in many cases, not to be found. Maybe he had hit her, caused bleeding, knows his DNA showing damage. Fear of getting blamed overcomes all other considerations. Can’t face it and media and citizen attention just too much so he bolts again. How reasonably can this be presented by his attorney? That will be the question. When and if he is found