Genealogy /
Am I of Polish or German descent? [10]
You are ONLY of Polish descent if you can trace your relatives to Poland when it existed as a nation.
No. It depends on ethnicity. In any case, Poland has always existed as a nation, just not always as a state.
a secondary citizenship
For citizenship/passports however there have to be clear rules, or Poland would potentially have an obligation to hand citizenship to thousands if not millions of people in other countries who don't speak the language and have to 'trace their relatives' in order to establish a connection.
West Prussian - German heritage
Remember Poland has never been an island and the modern borders are relatively new. Due to many, many factors, ethnicity has always had blurred edges here. Before the 20s/30s, not everyone (especially rural people in the borderlands) considered themselves to be Polish. Some did, some didn't; it depended on the family/individuals in question, however descendants of people who left over a century ago aren't a priority for the government when it comes to handing out citizenship that carries Schengen residency rights, access to healthcare/welfare etc.