Current day Marian being Zelena would be weird. That would mean Zelena was the one be frozen after eating the cursed ice cream, Zelena was playing mom to Roland and that Zelena was the one getting jiggy with Robin Hood. Also why would Zelena say to Regina “Yeah I’m happy for you to be with Robin”?
Well it would explain why Marian suddenly started freezing again, if it turned out Zelena did that to herself on purpose knowing it would mean Robin would leave town and separate from Regina in order to save her. None of Ingrid’s other victims randomly reverted to their freezing state like Marian did. And of course, it would make even more sense why TLK didn’t work between her and Robin. If they were to go the fake Marian route, it would also be a way for them to whitewash the moral issues Outlaw Queen faced following Marian’s “return.”