All just my own opinion …
If the information about the Fitbit not synching since Tuesday is correct, then for me that puts into doubt that Nicola was at the bench at all.
They can’t know she was with the phone or the dog when they were left there. She could have been taken from the top field, and then the phone and dog left at the bench. Specially if it was someone local or staying in the area and who could reasonably have been expected to be seen walking there.
It would have to be pre planned, but since NB followed a regular route, she’s seen in the field at 9.10
Let’s say called over by someone she knows near the Riverside caravan park to help with an emergency’
Incapacitated and bundled in a car / caravan.
Perp then puts Phone in pocket, dog on lead. Jogs or walks to bench, lets dog off and puts phone and lead down. Goes back to car/ caravan where NB is (either through fields or down road). Either takes her elsewhere, or maybe she could still be there.
Risky, especially the bit with the dog, but less so if it is someone known to NB.