Im just seeing it as; The wish- Emma’s wish was that she never became the Savior. That would mean no Dark Curse. And then what we will see in the AU is how everything would play out without the Dark Curse. Regina would still be the Evil Queen, if they explain why she hasnt aged, it only needs to be something like- magic is the best form of youthserum.
I dont see why this AU woudnt work? What facts other than the curse would have to be undone? I mean, yes, everything after the curse would happen in a different way, but that is the awsome and interesting thing about the AU.. and IMO they dont have to explain all the details, only the ones that are important to the current story.
They DOO need Baelfire to somehow have met Emma. Im confident they will have thought of some elaborate way of making that have happened- (lol!!!! does that sentence work? or did I totally screw it up??)