Sigismund III did not "introduce" any laws since he had no power to do so. In order to become law a bill had to be accepted by the Sejm, Senate and finally the king. And he was not a religious fanatic, like he is presented in the current popular history. Sigismund current reputation comes from popular books written by Pawel Jasienica in 1950-60s. However Jasienica fabricated lots of stuff. I mean really lots, he himself compared his works to novels. In real life Sigismund even allowed Protestant services to be held in his own royal palace. That's a long shot from a religious zealot like he's usually portrayed, right?
The Orthodoxes were treated the same like adherents of other Christian denomination, mainly literally all offices were open to them with the sole exception of the office of king himself. The Orthodox power in PLC waned because the Orthodox aristocracy gradually converted to Catholicism and got polonised. They did it by their own accord without the inference of the central government. In fact the central government was perhaps the last to recognize the polonisation and catholicisation of the eastern magnates. For decades documents sent to eastern Voidvodships were translated in the Royal Chancery into Ruthenian and then sent to the East, where it was translated back into Polish for the local magnates to understand them.
There was also the Union of Brzesc in 1596, in which most of Orthodox bishops severed their ties with Moscow and entered union with Rome. This event, portrayed in Russian historiography as a Catholic expansionism, was in fact done on the initiative of the Orthodox nobility and bishops who were fed up with Moscow's influence and her treatment of the Orthodox as a political tool. Only one bishopric (Mstislav) refused the union and was let alone, nothing happened to both her bishops and bishopric's estates.
You are overly idealising the situation with tolerance during the first two Vasa reign here. This has nothings to do with popular writings of Jasienica, just read Tazbir, Wisner, Augustyniak or any other classic or modern academic researchers of the 17th century's Polish-Lithuanian confessional history. Sigismund III did not even allow to bury his own Lutheran sister Anna Vasa for years, and it was only his son Wladyslaw IV who allowed a proper ceremony to be held in Torun in 1635, ten years later after her death. The reign of the first Vasas was full of the major Protestant and even anti-Jewish pogroms both in the Crown and Lithuania. For example, as in Vilnius in 1639 that resulted in pushing the Calvinist church, hospital and school outside the city walls in 1640 as per decision of the Sejm commission inspired and confirmed by Wladyslaw IV. A year before this the famous Socian academy of Raków was closed in Poland.
Regarding Brest union, you should really take in mind the closing and expropriation of the hundreds of the Orthodox churches that often were done against the will of the locals. As in case of the actions of infamous
Kuntsevych in Polotsk and Vitebsk.
More or less, yes. In the Deluge Protestants in large part turned out to be disloyal and threw their lots with the Swedes, the most striking examples being the Duke of Prussia (who was the first senator of Poland)
Duke of Prussia never had a place in the PLC senate. He was the vassal of the Polish Crown, yes, but that's it. Only the nobles of Royal (i.e. Polish part of) Prussia were allowed to send delegates to the Sejm as well as some Prussian cities (the latter without the vote only as observers). The first senator of Poland (and Lithuania) was the Archbishop of Gniezno (Primate of Poland).
and the Protestant branch of the Radziwill family. Afterwards it was not cool to be a Protestant and Protestants largely converted into Catholicism.
Except the thousands of the
Polish Brethren (Socians) that were expelled from the country in 1658 after they refused to convert.
While some political right of Protestant nobility were curtailed, mainly the right to sit in the Sejm and Senate, the was still a number of public offices open to them as well as all possible positions in the military (in fact Protestants were always over-represented in the PLC military). They economic and social rights of Protestants were untouched for all social classes.
Which offices exactly? Even the offices in the city councils (except the ones in Royal Prussia and Poznan) were solely guaranteed for the Catholics after the series of the laws in 1660s.
In case of Orthodoxes, there was not so much to curtail mainly because the number of Orthodox nobility were constantly dwindling due to ongoing catholicisation which was a voluntary process. By late 18th century there was no Orthodoxes of influence left. There were some Orthodox nobles scattered in some more remote areas, insignificant and without any clout.
I think it is quite hard to name such process "voluntary" when a group of people was deprived of a large portion of their political and civil rights (to occupy state offices) and treated as a marginal social group (named literary "dissidents" in the documents of the PLC era).
