Equality between Poles and Prussians is the only requirement, it can be a democratic state, aristocratic state or socialist state, with no time restriction for the PoD.
I think most likely pro-German Poles would convert to Protestantism, while anti-German Poles would stick to Catholicism, causing an everlasting Civil War.Perhaps having the Commonwealth go Protestant might be a good start?
Nope. Can't have OTL 'Prussian' without Teutonic Order-State, and can't have that without ever-increasing enmity between the Order and Poland, which will ultimately come to a head and result in one or the other dominating the region (check out my TL for---oh never mind), which will result in one or the other group being 'subject' to the other. I just don't see it happening.