I've just been catching up with the thread and here are a few thoughts:
One thing I've not seen mentioned is that in one of the pictures in this article, the tree by the bench has a sign on it that says "dogs must be kept on lead at all times". This doesn't eliminate the possibility that Willow was off the lead as I'm sure that not everyone follows the guidelines, but just thought that it might be worth flagging.
Missing mum said path was 'spooky' before vanishing without trace on dog walk
Sadly, I think the most likely scenario is that there was a tragic accident and she is in the river, but the mechanics are hard to get my head around.
The incline is steep but it's not a straight drop and looks to be almost separated in stages, so it wouldn't be a straightforward tumble if you lost your balance near the top. Unless she was already close to the very edge at the bottom, there would likely be skid marks or prints from her trying to brace with her hands as she rolled down.
It's possible that if she was listening to the work call she was doing what I do in those situations and pacing back and forth as she looked out across the river, holding the phone in her hand. If Willow was off the lead, perhaps as NB turned around Willow was underfoot and/or she lost her balance, flinging the phone out in front of her and falling down the bank.
An accidental fall is plausible IMO but the elements that would have to combine in such a short window to make this happen are a truly unfortunate series of events.
All JMO.
This case is so sad and I feel heartbroken for the family. Let's all hug our loved ones extra tight today.