I'm not sure that Joe had been abusing Ella, but I do think he was grooming her to tke her mother's place (all the looking after mum stuff, teamwork etc)
I think that the theory about young girls is right - hence the nightie on backwards, and anal sex so that there was the appearance of a flat chest.
BUT,, I think Juliet knew that as her pregnancy progressed and she was no longer what he wanted, then he would turn to Ella - hence her "I did it for you" comment