It's the same with Alsacians in Eastern France. Until the 1970 most of the population there, in der everyday lifes, spoke their Alemannic dialect, similar to the German spoken in Switzerland and the German state of Baden-Württemberg in the South West. They are definitely of German origin, in fact the area was annexed by French King Louis XIV. after the 30 years war. But it doesn't change the fact they consider themselves French and not German.
I think the same is true for many members of the German minority in Poland. Most of them are definitely of Polish stock, but if they choose to consider themselves German, what can you do about that?
All nations are "imagined communities" anyway. I would say more than 30 percent of all Germans are originally of Slavic origin and what do you think? How many Poles are of non-slavic origin: From the Pre-Indo-European original European population, Germanic, Celtic and Baltic?
You mean like Klaus Wowereit, the former mayor of Berlin?
Yes, his surname meant something like "Squirrel" in Old-Prussian. Just take the Lithuanian word for squirrel, the living language that is closest to Old Prussian. There squirrel means "voverė".