OP, what actually do you think the police should/could be doing?
Because, frustrating as it seems, I'm not sure, after all this time, what, other than someone coming forward to give a statement would be able to be done.
He's an adult now. If he was kept in the camp, and now isn't there, I don't think there's going to be any evidence left. And the community there seems to have rallied round the couple, so I wouldn't hold too much hope of them coming forward and telling.
If he's still there, well, he probably thinks he belongs there. He was too young to have any memory of before that time. If he's always been told he's their child, he may have no reason to doubt it. And even if he did, he may feel it is disloyal to voice it. if you look at these cases where a child has been abducted to adulthood, it isn't an easy decision to come forward.(Stockholm syndrome etc.)
It would be a very difficult decision for an adult Ben to come forward. Because if he came forward, and wasn't Ben, he'd be at risk of being rejected by the group.
They can't force the entire lot to come forward and have a DNA test unless they are accused of a crime.
If the police announce they're going to look into it, then it gives them chance to get rid of any evidence. Might actually risk Ben's life if he is there. For all we know, the police could be asking the media to keep that side down so they have the chance to investigate before the group prepare.
Unfortunately, I think the idea she's Ben's daughter is very unlikely. If he's not with the camp, then it's unlikely he's given his daughter back to them.
If he is with the camp, then it's likely the mother is too. Then the couple would have said "that's the mother, we're looking after her because she can't" and it would have ended there.