I say that the game take place solely on the Eastern seaboard, or only slightly in. The United States are HUGE, and if you ended the boundary at say, the Great Plains, that would be more than large enough to count for all of Western Europe. Another suggestion? put in Mexico! Having South of the United States fun with Catholic duchies, who might decide to go on a crusade against the Protestants. They might be allies with duchies and counties in Louisiana (large catholic population), or other catholic outposts in the US.
Also, the Deep South being not fit for human habitation without Air Conditioning? Who grew the Tobacco in the 1700's? The Northeast? No, the South might not have as high of a population as the North, but people can withstand 100 degree days (much like anyone living in the equatorial zone without air conditioning). Also, the South has much more fertile land than in the North, and they don't have to deal with the heavy winters that the Northeast does. Also, the main reason the Northeast was so successful in the 1700s was that it relied on manufacturing and TRADE, two things that aren't going to be as needed in the post-apocalyptic world. Big Ports are useless when you're focusing solely on North America. The only ways I can see the Kingdoms of the North prospering is using Old Age technology, and working to restart the factories of old, so they can manufacture things the South doesn't have access to. Otherwise, the Northeast will have no claim to fame.