The key criteria of a war that makes it a civil war is that it is between two opposing goups of citizens of the same country.
That's a necessary but perhaps not entirely sufficient criterion. Internal conflict can get pretty hot in all manner of political systems, but I don't think it's fair to trot out the w-word unless there's at least some serious checkpoints on civilian traffic if not outright territory seizures by the minority faction(s).
As far as which side is the usurper it is always the loser since the winner writes the history books.
Except for those horrible, occasionally decades-long times when things are a total mess and there is no clear winner who can physically and/or symbolically beat our delusions (memories) out of us. I've been trying to console myself about how ugly things are at the moment by thinking how truly awful it would have been to live during the Hundred Years War or the French Wars of Religion with 24-hour 'news' channels adding to the mess.
If you drive around in the US south it's pretty clear that many people are still fighting the Civil War or as they call it the War of Northern Aggression.
Please don't forget that many of us Southron folks who love the phrase "War of Northern Aggression" would also have been staunch, if terrified, abolitionists back in the day. We just don't like the damn Yankees and no-account cowboys from out West conspiring to label our region as the national homeland of ignorance. Reconstruction happened down here, but Jim Crow was nation-wide.