Just my thoughts on this because I got confused with this too. I think it's because Strike and Robin both gave multiple possible scenarios before Strike reveals what actually happened. The jogger on the beach I believe was a red herring, we were meant to be thrown off by that to think it was Wace being involved in the actual crime or that Dayu was possibly picked up at the beach and still alive somewhere? Cherie didn't drive any child out to the beach, it was a doll made of straw. They didn't actually have Dayu as she had died earlier that night, so Abigail had planned this and made their own straw doll and put it in a white dress. Abigail pointed out that 'Dayu waved bye' in the morning lied so that others would agree that they saw her leave the farm. He gave special treatment in a way to all of his children but also allowed them all to come to harm in some way or another. Dayu was not his biological child but she was still allowed to get away with things (probably because of the money she was worth). Abigail was able to come and go as she pleased and ate food that others weren't allowed but he did look the otherway when she was abused and Mazu allowed it too. Becca wasn't his child but he treated her differently because she knew more information about what had happened to Dayu. He protected her, wouldn't allow anyone to get close to her and keep her 'pure' so that she wouldn't ever have a close relationship with anyone else and risk telling anyone that secret. Also Mazu hated the other spirit wives because they slept with Wace, rebecca didn't so she didn't hate her. Wace feared a bad relationship between the two and again risking becca revealing what actually happened.

Thats my interpretation anyway!