I think some of this is the police’s messaging and communications. They do have to be open to other options than the most likely scenario. I think they were but in the past, the police have been guilty of “tunnel vision” that has led to mistakes on other cases.
On this one, Occam’s Razor applied and was seemingly proven correct, but I think, given the case attracted so much attention, they could have added some extra comfort on the media comms: we are working on the basis NB is in the river and will resurface as that’s the most likely scenario, but of course we are investigating all angles thoroughly.
Nicola’s profile as a hard-working and dedicated mum in her 40s struck a nerve with a lot of people. This was someone many people in the UK could relate to. I’m not trying to be glib here — for example, I’m a lesbian, no children in my 40s, and that essentially puts me outside a “normal” frame of reference and I don’t mean that in any disparaging way to the LGBT community I am part of! — but what I mean is Nicola was so very “regular” a person that a lot of people identified with her and that increased the press attention, internet speculation and worry.