I agree that any magical reaction that was taking place was created in and seeped out of SB. Certainly don’t think there was any magical force in our world reaching out to connect in the other direction. And if we never get an official explanation about it, I’ll be satisfied with assuming there was some part in the curse that provided for these things. There’s no way Rumple could have written all this type of stuff specifically into the curse, because he didn’t know the technicalities of the world they were going to. So an all purpose, “make sure the fact that we’re displaced beings from a magical realm remains undetectable to the rest of the world we’re in” clause would suffice.
But there’ll still always be questions like how supplies were delivered to town if no strangers ever come in and no one can leave. They can’t be completely isolated from and also integrated into the world at the same time, there’s gotta be a middle ground of some description.