The falling in the river from near the bench doesn’t make sense to me now given its depth at that point (3ft) & very SLIGHT current. She would have been able to stand up in it. For her to have fallen at such a force that she entered the water head first , then banged her head on rocks would have meant she would have had to positively hurtle herself down the embankment, enter the water head first, rendering her unconscious and thus drowning. That’s assuming she never put her hands / arms out to break the fall as people naturally do.
If she had slipped on her bum down the embankment and entered the water that way she would have been able to stand up and get out.
Also if the police timings are to be believed (phone being on the bench at 9.20am and Appointment Woman finding phone at 9.33am) appointment woman could have looked in the river and seen her if she had gone in mere minutes earlier?
But cannot rule out being in the river from a different location, or that Nicola never actually got to the bench. I’m sure the technology police have had access to details more, but for example my iPhone tells me I’m at least 50 metres away from where I am. ( that current location thing) and my car is .6 mile of a mile away from me. It’s not. It’s in the drive.