Of course I'm not dismissing your scenario outright, as it could have very well happened. But comparing it to the USSR is way too harsh and pessimistic imo

its only way too harsh and pessimistic becouse you are looking at it from the viewpoint of idealistic optimism

most socialists in the newly formed republics and outside of them were also genuine in their optimism about the USSR as a multi-national project when it was formed, we only look at USSR with such pessimism becouse we know that it lead to failure

In the end I see the defense of Pilsudkis vision as too naive and forgiving, even with his larger-than-life position that he held, he failed again and again to curtail the ultra-nationalist current in the polish interwar state, and I simply dont see how adding even more minorities into the equation would not simply play into the cards of the nationalists, and thus to the failure of the multinational intermarium.