I doubt that Regina expected Mary to “leave” exactly, I think she’s just expecting that because she can’t leave, she’ll die in the process of trying. I know we’ve seen other people try to leave and they’ve just been stopped short, they haven’t actually died, but Snow was the main focus of this curse, so I’d believe that an attempt for her to leave would in fact result in death.
There is something I’ve wondered about though, and I dunno if it’s something we’re really supposed to wonder about, or if we’re just supposed to accept that “it just is”, but somehow, someway, someone must be able to come and go from town, otherwise how are the shops kept stocked? Where does all the food come from? And the other groceries? Eg: It’s not like Storybrooke has its own toothpaste factory to keep up the supply, right? Where does the stock in the clothing shops come from? Where do the comic books we’ve seen for sale come from? Even if it’s all mail ordered, how do the packages get there? If no one from outside can enter town, then the delivery guy can’t exactly just drive into town.
So either we’re not meant to think about that too much and just accept that it happens somehow, or someone is actually able to leave town to go pick it all up and bring it back in. But they’ve made such a point of saying that no one can leave, that I’m thinking it might be the former.
Obviously they can contact the outside world though. They have the internet, and can make phonecalls. Even if Mr Gold can’t physically leave town, he was able to make some calls and pull some strings to get Henry sent there. That’s where my theory came from that perhaps August has been Mr Gold’s “man on the outside” so to speak, because he could have set that stuff up. Might have to abandon that theory though because the promo for the next ep doesn’t exactly make it look like August and Mr Gold are BFFs who have worked together for years.