If there were no Unia, there would be no OCA, no ACROD, a smaller UOC. It's that simple. Deacon Lance is correct and I suspect that when he checks in, Yurysprudentsiya will concur.
I'm not justifying the Unia, but history is cruel. I agree with the historians who assert that the east Slavs (not today's Russians though) would have been assimilated into the Latin Rite. But, who really knows, it's just speculation as we are where we are today and we just have to play the hand we are dealt..as people, as a nation and as a church or churches.
History is cruel, and cruelest of all to such historians. The records of the debates of the long-lasting 4-year Great Sejm (1788-1792) survive, and they record the panic of the Poles that the "Ruthenians", having failed to ever win over the majority of Sigismund's Orthodox subjects, were continuing to lose ground to the bishopless (by then) Orthodox. Finally the Metropolitan who sold out to the Vatican got his seat in the Senate, promised two centuries before, but only after all the Vatican's Latin bishops in Poland-Lithuania, and none of his co-conspirators got theirs, but the government had to not only allow but sponsor the formation of an autocephalous Orthodox Church for Poland. A funding debate for the Vatican's Ruthenians in the
Ukraine (to state the view of Warsaw) devolved into a spending bill funding, at the demand of the Polish Latin bishop of Kiev (yes, they never gave up on that dream, even though he was not in Kiev but in Żytomierz. Ironically, after the partition of Poland, he ended his days as the Vatican's bishop of Mogilev, the seat of the last Orthodox bishop in PL before Russia liberated it, set up as the Metropolitan of the Vatican's flock in the empire by the Empress of All the Russias), the setting up of a network of Latin Polish parishes for the Vatican covering Ukraine.
As for the OCA, I don't know. It wasn't founded among the Galicians, although they did become its core a century later. I do not know, however, how many of them passed through Bukowina, where a Ukrainian Orthodox Church of hundreds of thousands arose right under the nose of the Vatican's secular arm. Transylvania next door to Galicia and Slovakia never succumbed to the Unia, as you call it. Why would the East Slavs? A.C.R.O.D. I can't speculate.
As for the UOC, the majority of the Ukrainians in West Ukraine remain with the Orthodox (canonical or uncanonical), and have since before the Sobor of L'viv. The censuses of interwar Poland showed an increase of the Orthodox share of the population.