As ever, not much to add, except:
“He/she told me he/she was going away” is the mother of all cliches in suspicious disappearance cases. Anyone who says this goes straight to the top of the POI list in my book.
Additionally here we have a suggestion that she was meeting an unknown third party at the filling station - another old favourite, intended to introduce a new potential suspect in the case.
The phone is not the person - if person A’s phone calls person B’s phone it doesn’t necessarily mean that person A spoke to person B; person B could be in possession of both phones and simulating the call.
That said, I was interested by the claim that RR was due to attend the court appearance the day after BH’s disappearance. With the timing of the procedure apparently under discussion that night it’s not impossible that she may have made a call by way of a last effort to persuade him to attend - and events took a turn from there.
As to what those events were, my initial guess would be that BH was driven to the pond at gunpoint - probably from an initial meeting location and subsequently shot dead in the driver’s seat of her truck. Her body was then removed to prevent any ballistic match and the truck driven into the water on “auto creep” (too big for one or two people to push and no tyre marks of second vehicle afaik) in an effort to degrade/contaminate any potential forensic evidence relating to the occupant of the passenger seat.
What does this add? The removal of the body to avoid a ballistic match might suggest either that she was shot with her own weapon (indicating knowledge of its presence and location in the vehicle) or that she was shot with a registered firearm belonging to a third party which might be recovered in the course of the investigation.