As I say, if things actually go that far, none of this will matter to people. My guess is that the seceding states would repudiate the debt and leave the rump of the US to deal with it.

The break up of the USA would not be the civilised affair Brexit was. There would be real civil urest and potentially armed conflict along borders. As you point out, there is no legal, constitutional process for secession, so those who lead any seceding states would ipso facto be those prepared to disregard law.

The real danger of or to the EU is just now becoming apparent. Before Brexit, the French and British were the major military powers in Europe. With the UK gone, and taking the US with them to all intents and purposes, that left the French. But the invasion of Ukraine by Russia now has Germany rearming at top speed. Within a year, at most two, Germany will be back to being the military power it was in 1939.

The French will not like that, an armed Germany is something they still dread. Whatever happens in Ukraine, they will sooner or later begin to push for the Germans to disarm again. When Germany refuses, the EU will likely split along Franco-German lines, each one taking its client states with it. Another Franco-German war? Not likely, but certainly possible. If it does happen, one can only hope we Brits have the sense to stay out of it this time!