Why do you think there would have been a civil war?
Largely because of the way that Poland was heading in the late 1930's. Pilsudski was dead, and the Składkowski.government was turning more and more hostile towards minorities. You had educated Poles demanding (and getting) restrictions against Jews, you had Ukrainian schools/groups/organisations getting closed down, and most of all, you had personality cults building up around people in the government. There were also reasons for neighbouring countries to fuel conflict in Poland, and I think it would all have ended up in conflict. The Polish majority would never have understood life in mixed areas, and I think things would have just exploded at some point.