1. A baby can only be this magical with 2 magic parents
Emma Swan was born of non-magical parents
Cora was born of 1 non-magical parent, her father
2. The deal with Rumpel was only for their child. So why would Cora send any other child away? She could always pretend it was Henry’s. And it must have happened between the changing the deal and getting Regina
Cora gave birth to Zelena before she ever met Henry Sr.
I think the sexual tension is to throw us off scent. Zelena clearly has daddy issues. Rumpel can’t see everything, he already has explained that to Henry in New York. And when he is on a mission he can be very singleminded, so maybe that’s why he has not realised it.
given that she is going to forcibly make out with Rumple in 318, I don’t think it’s there to throw us off