I read in an interview once with Ginnifer Goodwin that Mary Margaret is the way Snow White would have been if she’d lived a very sheltered existence. So, if the curse is lifted, her true self comes back to her. I’m not sure if she’ll forget the last 28 years of her life, but that’s a very interesting and plausible theory! If the last thing SW remembers is the curse coming to FTL, at least she won’t remember all the pain that David put her through in the last year! Even better if Charming forgets his little David episode. If it doesn’t play out like you predict, it’ll still be very interesting for them to work through their two different sets of memories and identities.
"That’s how you know you’ve really got a home. When you leave it, there’s this feeling that you can’t shake. You just miss it." Neal Cassidy