I think there is one factor missing: While it might be true that many of us are reading from left to right, our usage of UI is not necessarily defined by the sa

I think there is one factor missing: While it might be true that many of us are reading from left to right, our usage of UI is not necessarily defined by the same patterns as reading. When we start using a UI we certainly read every button before clicking on one, but after a while we just want to find the correct button. And in that case it might be helpful to place the button for the primary action right next to the bottom right corner, as that corner is one of the places we start looking for the button. With the OK-Cancel order, the OK-button would be hidden behind the Cancel-button. Nevertheless, this factor doesn't change anything about your conclusion. I think your argument is very balanced and I agree with your conclusion about consistency being the most important factor.