I thought how Jefferson got the apple was pretty obvious, the Hat isn’t just a bridge between worlds but time & space so that’s where the hat opened the portal to the past when Snow bit the apple.
So, wouldn’t it make sense that if Emma does have a “seed” of FTL magic in her that is awakened by her belief, that if they could make the hat work they could go back before the curse was enacted and stop the Queen? That’s what I’m thinking makes the most sense.
I just think that the curse was gonna happen & there was no preventing it. If there was something else TBF or Rumple would have said so. Going back & undoing the curse would mean Henry never exists, so why would Emma do that?