I don’t think it’s a double bluff. I think Zelena is Cora’s eldest daughter (based on the photo from Rose McGowan’s instagram of the birth in what is clearly NOT a princesses’ bed)
The bigger question for me… how did Cora’s body get back to the EF? She died in our world. Regina buried her in her vault/family mausoleum in Storybrooke, didn’t she? So… it poofed back to EF along with them? Complete with a blood magic doorlock?!