The casualty figures are interesting. Germany is the closest analog we have for a rump US, with roughly equal population in 1914. Assume for a moment that the above figures (1M plus for the US) are for deaths. Germany had over two million military deaths in real life. Subtract another 20% from the fourth year of war (deaths tolls tended to be highest in the first years of the war), and we're now at about 1.6m German deaths, versus maybe 1.2m US dead.
(I'm trying, unsuccessfully, to find WWI casualty figures broken down by year.)
The Red Rebellion is another hobby horse of mine. I believe it was mentioned at one point that roughly one in ten Negroes rose up, which is probably about 1 - 1.2m. Given that the Red republics were crushed, casualties well over 50% wouldn't be out of the question. Hundreds of thousands of Negroes may have been killed either in the fighting or in reprisals.
How many whites were killed? Even though they had the upper hand for most of the fighting, they probably took terrible losses nonetheless. And that's not counting white civilians who were killed early in the rebellion, when the Reds cleansing their territory.
Given that we can infer "less than a million" Confederate deaths from the above quotation (800K), this probably means that Rebellion deaths were not included in the war dead. Overall Confederate deaths - Great War + Red Rebellion - may very well have equaled the US's (who probably didn't count Mormon dead, for that matter). The US likely has twice as many people overall as the CS - that's a terribly high death toll, roughly 5%, give or take a point. Very few major combatants lost more than 4% in the real Great War, even with a longer conflict.
I've theorized in the Canada post that they mobilized, and lost, roughly in proportion with Serbia, which lost about 15% of its people in WWI. That would equal over a million Canadian deaths as well, if my guess is accurate. That's about fifteen times as many in real life.