I cannot imagine that it ends in a civil war. Civil wars happen when you have
1) a lot of young men,
2) who have little to lose, and
3) who are used to living a life with not much more comfort than that of a soldier (maybe even less)
The young generation in Catalonia is small, they have a lot to lose, and they live very comfortable lives.
I don't see them taking up arms. Maybe they will take to the streets for a few days, but they are not willing to fight in great numbers, or give up studies or jobs.
In the last civil war in Spain or the one in America or the ones in the Middle East, those conditions were present.