I think initially police insisted Nicola had had a fall into the river due to the time window suggested by sightings of her - it's not very much time, if you believe the times of the sightings of her are accurate.
My feeling was that as all the sightings of her were estimated times ( I read that she was seen at 9:10, and then someone found her dog, unharnessed and with lead off at 9:30), those times were up for debate. It may be that were accurate - you can check when your texts came in after you saw your friend etc without looking at the clock at the time etc.
Due to the short window, the police perhaps assumed a fall, quite reasonably, as the chances of someone attacking her and abducting her in such a difficult-to-reach area within 15 minutes unnoticed were slim.
On the other hand, what if the phone was dropped when she was potentially attacked? Someone could have gone back and put it on the bench. Or, if it was dropped, just found it and put it there - as people sometimes do with gloves or hats found on walks.
My other thoughts about her disappearing at that time are more to do with who knew she was there, and who knew she was on her phone. Do we even know how long she was up there?
If someone knew she was logged into a Teams call and also where she was, it would be easy enough to plan to put the phone there after something dreadful was done. Perhaps she never reached the bench. It would explain the lack of forensics. I have questions about the person who tied up the dog they didn't know.
Geographically, this suggest to me the police should be inspecting the possible exit-points. Properties nearby have been looked at. What about forensics at nearby car parks, roadsides etc.? Where was Nicola's usual entry point to a walk, and when she let the dog go where would she have logically wandered to both hear a call and see the dog? Where could she have been approached without seeing the person? Why wouldn't she run if it was someone unknown to her?
My feeling is she was either approached and unaware of that person approaching her, or else it was someone she knew and that's why she wasn't panicked. Whatever happened next possibly happened out of sight of the dog, and that's why it wasn't stressed. As far as the dog is concerned, Nicola is alive. For all we know, she may be?