"many Poles, who were born on territories, that used to be German before WW2, and now belong to Poland. And - as far as I know - it's quite easy for them to get a German passport"
Are they ethnic Poles or are they Poles of German ethnicity? Its pretty easy if they are the later - many Volksdeutsche from all Eastern countries have and continue to migrate to Germnay; I wasnt aware of it being possible for Poles from former German lands to. Unless I guess they were Poles who were formerly German nationals in the pre-WWII days? There are quite a few of those still around I guess.