Apologies if this has been mentioned as a possibility before and I've missed it. But the posts that I've seen doubting that she's fallen in the river all seem to mention that her dog would have jumped in too or that the river is too shallow for her to get into trouble
MOO but I think if Nicola fell in, then she wouldn't have necessarily panicked right away. She would have been a bit shocked and cold but if she could stand up in the water as suggested, then she would have been calm and just thinking how to get out. She most likely would have told Willow to wait/stay on the bank, because she would have presumed that she (Nicola) wasn't in any immediate danger. She wouldn't have called out for help for the same reason. After realising she couldn't climb back up the bank at that point, the next logical thing would be to wade along the river's side to find a point that she could climb out at. If she headed in the direction of the bridge and told Willow again to wait, then once she was out of sight, the dog would have fretted and could have tried to follow her on land, which would be through the gate. Which could explain why Willow kept running back and forth to the gate, agitated
Then, IMO, one of two things could have happened: either, while wading down the river, Nicola unexpectedly hit a deeper, faster flowing part, and got into trouble. Or by the time she found somewhere to get out, she was so exhausted/injured/hypothermic that she then collapsed in deep undergrowth and has yet to be found. My point being that just because it's unlikely she would have struggled in the water at the point where the bench is, doesn't mean she didn't move down the river and struggled elsewhere. Like I said, just my opinion
Just to add, a man in his twenties drowned in the river near me. The river is a bit bigger and it was about 12 hours before it was realised he was missing, but it was 5 months before his body was found by dog walkers, less than half a mile from where he was presumed to have fallen in. So I don't think it's strange that Nicola's body hasn't been found at this point
I'm still keeping my fingers crossed that Nicola has just run away and is somewhere safe