I have been reading a bit more about Polish history (very interesting, and not at all what I expected) and I have a few questions and things to run past you.
1. It seems pretty clear to me that from 1419 the Polish monarch ought to be the Sejm, not whichever elected king sits on the (relatively) powerless throne.
2. The game needs to adopt from the AGCEEP the Polish events related to the concessions made by each king prior to election. Alternatively, someone needs to write up an alternative set, but this does seem like a lot of work.
3. If Poland is re-formed through the religious-based revolts under Kowalski, there needs to be a struggle over the future of the Sejm. There will be many szlachta who will want a return to the Golden Liberty, and many of these individuals will have bankrolled the armies and might expect to get their way. Plus, the szlachta is about more than just entrenched land ownership, it's also a cornerstone of Polish culture in the period. But there will be many who feel it important to limit the power of the magnates. For others, the reformist doctrine forces the Polish state to look more closely at the rights of peasants.
For this last point, I'd like to suggest that it gets simplified, depending on the time it happens.
If Polish emerges during the life of Jan Zamoyski, then Kowalski will be likely be outmanouvered politically. Although a member of the Sejm, Kowalski will be primarily a military leader, at least until Zomoyski's death.
Zamoyski will negotiate a leaner, meaner Sejm, abolishing the old liberum veto and forcing the Senat to become an elected body, essentially the executive of the government, elected from within the Sejm.
On Zamoyski's death, Kowalski will get himself elected to the Senat and begin concentrating authority there, using the threat of foreign invasion as the premise for increasing powers. Kowalski's goal will be to have himself elected king, although he will be scuppered in this goal: the best he can acheive is king during his own lifetime, as an honourary title, with the kingship being abolished on his death.
However, if the re-formed Poland emerges after Zamoyski (1606 onwards) then we find the principal political leadership weaker and Kowalski will have an easier time of things. Poland will be less-well served, of course. Kowalski insists on an elected king, and that the country essentially return to its original 'constitution', with the exception of the liberum veto. However, does not get elected himself initially and remains at odds with the Sejm, focusing on campaigning. With the death of the king in 1631, Kowalski is elected, but demands a pacta conventa which grants the king numerous executive powers. near his death Kowalski forces the Sejm to pre-elect his heir under the same conditions. Later, with the heir's death, the kingship is returned to a mere figurehead.