In Kachalsky, at any level, the court will first look at the case and consider the outcome of granting Gura his desired remedy. If this remedy includes citizens carrying loaded firearms up Seventh Avenue, he's gone too far, and he's going to lose.
While I have no doubt "public safety" will be the central focus of the city's arguments, the Court is ruling on a larger issue. If they rule in our favor (which I can't see them avoiding, at least to some extent), that ruling will
bind the island as well as the rest of the state.
The idea of severing NYC from the rest of the country is a bit silly.