I really couldn't say how I would have reacted to being treated as a prime suspect in that case. I would have been going out of my mind, franctic with worry I am sure and if I functioned at all , it would probably be on auto-pilot. Who knows how sensible we might have been in their shoes and whether we really would have accepted being interrogated as a matter of course or just gone totally beserk.
As to closing the investigation, I wouldn't like to be the one to make the decision. I assume most people still hope she'll be found but it is true for parents who have not received this level of support for a missing child, it must be hard. Can't blame the McCanns for doing everything they can though.
Terrible business