This was my initial thinking too but now I wonder if that is just what she wanted people to think. There is such a lack of evidence for any scenario that it feels planned.
I do find it hard to believe she just left Willow free as anything could have happened especially if she could get out like the SIO suggested and be up on the road. No owner would take that risk and I can only think it's because she knew someone would find her within minutes. She sets the scene, hides nearby, and then leaves on the path past the weir or possibly by crossing the river where it is shallower. She could have hidden a rucksack the day before on her way back from her work meeting. The zoom call could have just been a cover for being late back. Change clothing, even using a wig. Walked away from the area whilst everyone was searching the fields and river. She could even have dressed as a jogger with a hoodie or similar and jogged off out of the searched area.
It's concerning though that if NB wanted to leave why would she need to go to such extremes to plan it and not just walk out like others have said. I know she may not be thinking rationally, as we don't know the extent of her struggles, but this case just gets weirder by the day.
Of course, she could also have taken her own life either via the river or somewhere else or had an accident and be in the river. 3rd party involvement feels less likely due to the timeframes but frankly, in this case, who knows?