I'm confused as to how you end up with this conclusion.
The Secessionists started the war. If secession had been peaceful then I could see a remote possibility of secession be legitimized. However, the Confederacy started the war, and in most realistic Confederate victory scenarios they end up bringing a foreign power into it. This will irrevocably lead to Secessionism being irrevocably associated with treason, discrediting it in the eyes of most American citizens.
And there is also the fact that the C.S.A. seceded to preserve slavery, further tainting Secessionism in the eyes of American citizens.
The Mountain Meadows massacre certainly didn't win the Mormons any friends. Nor would the Mormons have been better off had they opened openly fought with Union troops. The public would most likely have a problem then regardless of the the opinion of Buchanan.
You seem to be missing my point. James Buchanan did everything possible to enrage the Mormons, but nothing came from it. You need two to tango, and Brigham Young did not want to tango. The Mormons will move from the United States instead of fighting Union troops.
I'm aware of that. I was however hoping you remembered who Maximilian was backed by. And that would be Napoleon III of France who invested a sizable amount of cash and soldiers into Maximilian's cause. Having a friend in NA would have been of immense value to him.
Napoleon III did not control Frances military budget. Those that did were directing funds elsewhere because they hoped to curtail Napoleon's adventurism. France's OTL involvement in Mexico was already pushing the budget because of this.
No but you clearly don't know who the Metis are I see. I'd recommend reading up on that if I were you.
Yes, they used modern technology. Modern technology does not equal industrialization. Even during the Civil war the Union was an industrial power, and the first one to fight a truly industrialized war, so if the British Empire manages to best it as completely as you seem to think it will, they will have no problem with the Metis.
As for two Germanys... well not really. This is all assuming that the US still gains great power status (nowhere near guaranteed), doesn't have major political issues after the war, isn't fighting a war on two fronts and the British are really stupid.
1. The United States will still receive the majority of Europe's immigrants. Its internal economic growth will still fuel its continued industrialization. It might not rise as fast, but it will rise. It is too late to stop that.
2. The American Civil war
was a two front war. The United States will have the manpower and the logistical ability to fight a two front war on the American continent. You also seem to think the United States will be fighting alone and not look for allies against the British Empire, even though they will find natural allies in mainland Europe (Germany, Russia).
3. The British Empire did a lot of stupid things between the American Civil War and World War I in Our Timeline. Having the British Empire support the Confederate States is likely to exuberant these bad decisions.
Itself as a power. Not to confusing.
Redeem is a loaded word associated with morality where I come from. In this scenario the British Empire would have just fought a war in support a slave power, against a power that was popular in the eyes of the masses. From my vantage point, the only thing I could see was the British Empire's need to redeem itself in the eyes of its own people.