I've never played as America (ever in a HOI game ), but I don't think Canada declares war on the US during the civil (in pretty much all cases I've seen), they just take New England and/or Alaska sometimes. Mexico will occasionally attack the Pacific States and the other rebels (I've seen them take over basically everything of worth west of Louisiana, including Alaska).
Canada does usually take New England and Alaska, but occasionally I've seen them try to make a mad dash through the top-center of the U.S. and end up with pretty weird borders.
It's actually an event. The player of the "traditional" United States can either chose to confront Canada about the annexation of New England and Alaska and declare war, or give up New England and Alaska without a fight, in which case Canada tends to be an ally of the United States and war is averted.
Uphold the Constitution when it comes up.
In case it wasn't obvious, America First United are the Nazis. So I generally don't play as them. If somebody could explain to me how they aren't the Nazis, I'd be glad to hear it.
Combined Syndicates of America vary based upon the player. Remember they were formed by multiple ideologies. So after winning the Civil War, they can chose which ideology to follow. While they certainly can be Stalinist Communists, they can also chose the socialism common in modern-day Europe.
And for those who don't like either of the above, and don't like the "traditional" United States, note there is another major faction in the Civil War to chose from (no spoilers, but they're basically fighting to be left alone by everyone else).
Or we can be Hawaii, but that is the sort of thing we'd have to be planning already in order to achieve (basically like a micronation, except we could do a couple things now that would "beef them up", but it mostly involves exploiting knowledge of the later events in this mod).