I'll take a bite at the question you posed, "why anyone in the Northern states wants to be part of a country that includes the Southern states and their people."
I suspect that most people, then as now, view their country as the whole enchilada. "From sea to shining sea," as the song goes. Northerners probably didn't think much of, or much about, Southerners per se. But their country, the United States of America, included ALL the states and territories. And no one has the right to break up their country. That, I believe, is essentially how the Northern folks saw it, and it's not much different than the way people feel today. If, for example, a weak president acceded to a Hispanic secession of the American southwestern states, I doubt you would hear much concern about "states' rights", even in the South. Every patriotic American would demand that a president, who didn't do everything in his power to preserve the territorial integrity of the United States, be impeached for treason; and they would support -- demand -- that military force be used to return the territory to the United States.
Now, folks who don't know the difference between an analogy and a comparison aren't going to understand the relevance of what I just said. But most of them are adherents of the "principle of shifting principles," so discussion with them is by definition pointless anyway.
Once again, thanks for a very good post.