@DigOutThoseLemonHandWipes
I think Ameral's determination to pin Joana then Madeline on the parents was because he didn't want the area's tourist trade harmed by people knowing there was an uncaught child abductor/killer on the loose rather than this being in direct cohoots with the perpetrator(s).
To me that's pretty horrifying in itself though! If he genuinely thought the parents were responsible, I could understand his actions much better than if he thought innocent parents of murdered children rotting in prison while the perpetrator went free was an acceptable price to pay for the tourist trade.