So according to the DM the police say there is no evidence she fell in the river.
Well yes, we already know that and have done for 10 days now. Well done DM! There is indeed no eye witness or forensic evidence to confirm the working hypothesis that she fell in the river.
In the absence of that, the available evidence (witness sightings, CCTV, dashboard cams) is all pointing to the fact that Nicola did not leave the river bank and that nobody joined her there.
Hence the working hypothesis that during that 20 minutes if she wasn’t seen anywhere else, and never left the river bank, and wasn’t joined by anyone else, she must have fallen into the river.