I shared this theory in the chatroom, but I think somehow Zelena changes the past so that Regina was never born. In the sneak peek for episode 31, Zelena says her problem with Regina was that she “was born.” I think Zelena wants the life she feels she was robbed of.
If Regina were never born then Henry wouldn’t have been born either.
Rumple needed Regina to cast the dark curse. If Regina hadn’t been born, she never would’ve cast the dark curse, Emma would’ve grown up in the EF. Emma never would’ve met Neal and therefore never would’ve conceived Henry.
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"That’s how you know you’ve really got a home. When you leave it, there’s this feeling that you can’t shake. You just miss it." Neal Cassidy