I see and understand your point. And in any other country in Europe at the time, except the papal states, this plan is solid.
But, your problem again is the magnates, they aren't obligated to convert and in the rules of the PLC lets they maitain(or not) the churches and religous personal how they like so a official convertion to luthenarism not mean that the church lose any power as was the case in England and Germany
Yes they can upkeep the Catholic Church in their remains, of course that means they don't get parts of the Church's properties. In general the Kings split them between the nobility and themselves. Which Magnates do you think will do best, the ones which expands their holdings or the ones who doesn't.