Is the horse surviving really ASB? I mean, if there was any chance of it surviving the ice age, it wouldn't be ASB. Horses aren't exactly megafaunga in the sense of giant sloths and camels, and of course mammoths.
In our world, there were supposedly superior circumstances for development in the old world, which led to it eventually dominating the new world. I would increase the number of people who moved across the Bering strait, as well as making a few species like horses or mammoths survive the ice age.
I agree, my second choice would be to make sure that extremist communism never takes off. Europe already had people advocating for socialism who didn't believe there needed to be some violent, "glorious" revolution that resulted in an entire century of mess.