JMO
We on the outside do not know who these witnesses are. We do not know if NB fell in the river, we equally do not know if NB came to harm or foul play. IF, NB is not in the river, and it is ascertained that foul play is at play here, at what point will these 'witnesses' be more closely scrutinised and questioned?
Just because a witness, or anyone for that matter, says something, does not necessarily mean that it is true. Just because you live in the same area as someone, does not mean that you have to like them, or that you even like them at all.
This does not have to be overly complex at all. All it would take is the involvement of someone unhinged, maybe jealous of NB career, good looks or family, who was familiar with NB routine, (to be fair, look how much information us complete strangers have managed to find out with regards to her routine), for someone to be familiar with the local area, entry, exit points, any CCTV that does or does not work. If multiple people involved, then making NB 'disappear' becomes considerably easier. Especially if any perspective harm doers, just so happen to be any of the witnesses.
One thing is certain. What happened to NB, the answer or answers to that question remain firmly in the area. The answer to that question has never left the area. Either it being the river, or a 3rd party.