I personally believe the Curse is like a “program”. It’s run by an A.I. mostly but if Regina wants to override the system, then she can. I do not believe she individually wrote “everyone’s” story, only those of people she was particularly interested in.
You also could question if the Inifinite Forest is a bubble universe with no “way” out but magic then the mirror could see into that I think. But since it was a sub-universe really that may mean nothing.
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