This is why I was musing in the previous thread about whether there'd be value in putting something similar to a body into the water, to see what it typically does. The bench where the phone was found is (I think the police said at the presser) about 14km from the open sea. At what point would you expect the current to move a body that distance? What if bodies routinely sink out of sight and then take 3 weeks to reach open water? No sightings in 11 days could be exactly what we should expect.
I would discount any theory that involves anyone trying to abduct both NB and the dog, or abducting her and going back for the dog. If you want NB, the dog's just a nuisance that's going to bark and will betray your location even more noisily than the smartphone. You'd want to lose the dog.
Stealing dogs, especially purebreds, is a thing though. Is this type of spaniel valuable?