I've not quite abandoned the theory that NB ended up in the river after a tragic accident, but after the PA interview I've looked at what could be a plausible alternative.
NB is last seen by a witness in the upper field at 9.10am with her dog. She's not seen again. Her dog Willow is found alone near the bench at 9.33am precisely we are told. That is 23 minutes. A lot can happen in that amount of time in a small area. For the sake of argument, let's say there is a predator there. Perhaps he had worked out NB's movements over many days (PA inferred her movements were almost like clockwork). He forces NB and her dog off the field and applies restraint. He then has enough time to march the dog to the bench, pose the phone, untie the dog, and leave the leash and harness on the ground. The dog would not follow a stranger back across the field but would stay by its familiar harness. He then returns to NB, and then the abandoned dog and phone are discovered. He would then have many days to work out what to do next as the Police were fixated on the river and the tragic accident theory.
If we can accept that as a plausible scenario then I would be searching all of the caravans and building at or near Rowanwater and looking into the backgrounds of everyone who was known to be there that day or who had access and could have been there. Just MO.