Unfortunately, turning off historical focuses doesn’t prevent the AI from doing strategically dumb things. If the AI picks a focus that allows them to declare a particular war, they tend to declare right away. The problem is that the national focuses seem to route around the AI’s logic for who to go to war with.
As for divergence, I don’t think it goes too bad. I played a game as Canada with historical focuses off. Poland, Romania and Czechoslovakia ended up banded together in their own alliance. So when Germany demanded Sudetenland, they went to war with those three countries. Since I buffed AI Germany, they ended up winning that war handily, and subsequently declared on the low countries, kicking off the war with the Allies. After trashing France, Spain joined the Axis and they ended up invading the USSR in I think 1940.
So that’s kind of an alternate history, but not too crazy. The problem as I understand it is that a lot of wars are the result of national focus trees, so things that made sense in the original timeline maybe don’t make sense given the new situation, but can happen anyway once the country in question gets around to activating that focus.