Thing is, much of Belarus and western Ukraine
was converted to eastern-rite Catholicism, but gradually converted back again after the partitions as the tsar's started to rely on Orthodoxy as an agent of their regime. By the 1870s, Greek Catholicism inside the Russian Empire was tiny; those millions of Ukrainians live in Galicia, mainly, where the church became first good buddies with the Hapsburgs and latterly a national institution (in fact the Roman Catholic Poles, after WW1, found Greek Catholic Galicians much more intractable than Orthodox Volhynians).
My rough sketch would be something like kneecap Muscovy starting in the 1500 or 1600 hundreds; the PLC keeps Smolensk and doesn't become quite such a mess; continue to kneecap Muscovy; eventually Poland breaks up in the *age of nationalism and you end up with a great big East Slavic Eastern Catholic state including Smolensk, Belarus, and much of Ukraine.
To the above: I don't really follow. What period are you talking about? There's been an actually extant Belarussian political entity able to have disputes with Russia since, ah, 1991. Belarus (and Ukraine)
were deliberately converted to Greek Catholicism by government policy at one point, and as a matter of fact they
didn't take it too well at first.