Well, what is the date for "after the deluge"? 1660? 1667? Looking for the term gave me both possibilities.
If it´s after 1657 (1660), the Duchy of Prussia is no longer a Polish fief.
And I wouldn´t call Elector Frederick William, the "Great Elector", completely irrelevant and powerless? Rebuilding the state, creating a standing army, inviting the Huguenots...
The religious differences between Catholic Poland and Protestant Brandenburg might be more awkward. Same for Protestant Sweden and Catholic Lithuania. So the "split" doesn´t quite make sense?
Not sure what your point is here.
While nominally Prussia stopped being a polish fief in 1657, it
de facto remained one, paying tribute until 1700. The "Great electors" greatness came at the cost of depleting Brandenburg's population after he indirectly brought it to ruin by remaining so staunchly calvinist, 'inviting' both protestant Sweden to loot it and later the catholic league to do the same. Growth by percantage is hardly a problem once you bring a country to rock bottom.
What 'split' are you mentioning? Polish and Lithuanian? Seeing that the Lithuanian nobles were traditionally disloyal, willing to side with anyone who at the moment offered the something, I doubt they would think twice about backstabbing Poland which btw. they did OTL - before Sweden got its ass owned by the Poles, causing the Lithuanians to revert back to the Polish side. And given Poland's traditional religous tolerance, a King of non-catholic religion isn't that hard to imagine (not sure if catholicism was required, didn't orthodox tzrs try to get elected fromtime to time?).
I simply don't think a Poland-Brandenburg union would hold for a long time, both because Brandenburg would be overpowered by Poland in this deal, as well as Poland not wanting to be any closer tothe HRE. IIRC Sweden did try to introduce its own king to the Polish throne during the Great Northern War and he ended up having to flee really fast. Doing the sae earlier would probably give same results.