New here but long time lurker.
I think I tend towards the view of the police. The simplest answer is usually the correct one. It seems plausible that she headed back to the bench from the upper field and let Willow off the lead. Having had Springers, I know they like to run off and don't always stay close by. She probably sat down to concentrate on the meeting. She might have heard Willow barking in the distance, stood up to look out for the dog as she knows that there are issues with dogs off of leads, then turned her ankle at the top of the bank. Easy to do, she's wearing wellies. Willow comes back and has no idea where she has gone.
It's quite the tumble. It doesn't matter how strong a swimmer she is if she's dazed and has cold water shock, plus is wearing jeans, a coat, and has undercurrents to fight.
The fact that they haven't found her in the river yet is meaningless IMO. It's large, has pockets of weird depth, and has currents. I think the private search company has created a lot of unnecessary confusion. In my line of work, I deal with pushy experts all of the time and they can be wrong sometimes, no matter how clever and self-assured they are. It rings alarm bells for me that a person of this level of experience would have the audacity to say that anything is categorically true or false.
It's a horrible case and my heart goes out to the family.
MOO