I'd assume the phone had fallen out of her pocket so I'd pop it on the bench, and that Willow had gone off on a jolly and Nicola was looking for her.
I'd have secured the dog if I could, and carried on, if I was limited on time. And of i knew someone that could get in touch with her husband, I'd contact them. The finder was entirely reasonable. Who would have assumed death or abduction somewhere familiar, safe, in the morning? You have to look at all the context.
It's very easy to criticise with hindsight!
I still believe the police that they have firm evidence that Nicola has drowned but they're not telling us all they know.