Rumple has a knack for knowing what people want before they ask him. He could have foreseen someone (the EQ?) wanting a dark curse. Created it to his benefit and by having the EQ enact it he forgoes the consequences of when it is broken. He returns to human form in FTL and destroys the EQ at the same time.
I think Maleficient knew about the “void” because she read/studied the curse even though she didn’t enact it. I think Maleficient understood the curse better because her judgement wasn’t clouded by vengeance. She has a more objective outlook than the EQ.
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