I think NB's partner cannot do right. I'm echoing another poster's point here, but if he gave a statement or appeal, every single flicker of his eyes, every shadow of an expression, every syllable he uttered would be offered up as "evidence" of something or other. Given the circumstances here, and the understandable desire for a breakthrough of sorts, it would be open season.
By way of example, the posts on this thread went swiftly from "why is PA not searching?" = suspicious, to, '"Oh, wait, he did search" = suspicious.
PA is likely to be following police advice anyway, one would think, and focusing on their daughters' needs primarily. I cannot fathom how someone in such an awful position manages much beyond basic functioning and providing love to his children. Throw suspicion his way was always going to happen though, and he will be horribly aware of that, alongside everything else.