Imagining a Russia that accepts the use of Latin when it has a perfectly servicable alphabet of its own and no great fondness for being forced to adopt foreign ways is a bit much, though.
Its not impossible, but there would need to be a good reason for it.
Eh, it's been done, though generally later in history (Romania used Cyrillic until the 1860s).
This however:
MSZ said:
Why wouldn’t the Russian boyars accept a union in return for more power and money for them? OTL you had Russians offering the Crown voluntarily, so the “doesn't want to be merged into” part certainly isn’t a Great Patriotic Anti-Polish Movement, universally supported by all Orthodox peoples.
Yet that's what the Second Opolcheniye ended up being, in the end.
I think you have to realise that the Polish state has NO MEANS of granting the small nobility or gentry servicemen anything more than they already have, and that the Russian Great Nobles have generally been thoroughly broken compared to Polish and Lithuanian magnates. As witnessed by the reigns of False Dmitriy, Shuiskiy, and the Seven Boyars, the Great Nobles of Russia could hardly hold on to the country, much less control it.
Their rule, and that of the Intervention, was ended by a movement led by a petty noble and a wealthy merchant. Russia's lower estates had much greater participation in the consensus of running Russia than PLC's lower estates.
This is almost damning to the Russians given how things turned out, but any 1600s PLC-Russia union will need to win the hearts and minds (and purses) of MANY many MANY Russians, as opposed to just a few Great Nobles (who were the one sector who could really benefit from PLC rule, and did historically try to capitalise on it).
Or to put it another way: Russia's early centralisation and modernisation actually hindered its plasticity in the following centuries. There's a very strong parallel to the Ottomans, and for good reason. They had trouble reforming too.
EDIT: And this is not to speak about Siberian "free settlers", or Russia's client/allies in the Steppes, or the Cossacks. How would a PLC-Russia deal with the inevitable Bolotnikovs, Razins and Pugachevs (who will be based on the Volga/Ural), when they already had trouble with the Khmelnitskys and Bohuns in Ukraine?