I can't keep up so don't come for me but if the husband of the lady at the caravan park has a gun and the dog was off the harness, is there any possible chance Willow managed to get into the caravan park? Perhaps Nicola has followed and he's shot Nicola by accident (instead of the dog)?
The wife finding the items. Them being placed on the bench. It feels so set up, as if the person wanted you to believe she'd fallen in the river.
The CCTV conveniently being broken. It's all a bit suss (like a poorly written crime TV show).