Having previously stated my belief that "something untoward" occurred in the vicinity of the bench, for me the two things throwing a monkey wrench into formulating what happened to Nicola are - the phone on the ground, and Willow racing back and forth between the bench and the gate.
Upon reading through so many thoughtful posts on the subject, it is indeed not beyond probability that what occurred was simply an accident. Nicola was placing the harness on Willow and, for whatever reason, something went awry. During what ensued, the phone could have been dropped, or knocked aside. Willow might not even have been aware that Nicola had tumbled into the river. And, because Nicola should have been at the gate at about this time, Willow ran back and forth, expecting to see her, doubtless becoming progressively more agitated when she did not appear.
My reasoning tells me that Nicola did not take her own life. For the sake of her family, I fervently hope this is so, and that there was no other, malevolent, scenario in play.
At this point in time, speculation is redundant. In the best of all possible worlds, we will eventually have answers to most of our questions, and our minds will be settled to at least some positive degree.
God bless Nicola and her family. And all of us too.