Perhaps a good solution is to divide the country by counties rather than states. Then the vast swaths of red America wouldn’t have to go anywhere, after the blue cities are excised. There will be only a relatively few purple counties that will have to do something. Maybe that’s where the embassies will go, and their residents would be dual citizens.
We either have a “clean” secession state by state or we go to war. I prefer the former over the later.