Hello from Munich-Bavaria!
Just read your interesting story and maybe I can contribute a few ideas.
My ancestors according to what my grandmother told me in the later 1940s and early 1950s were - about as far in the past as in your case - a French family "de Pomplune" (one son married a Dutch girl named "de Graaf" (or de Graf). They both moved to Schleswig-Holstein, had children, one daughter married a man called Langenstraß (long street). The grandmother of my grandma was a "de Pomplune".
Now, according to my information, "de Pomplune" was a name for a group of Spanish people from Pamplona, Protestants, who fled from Spain to France and lived there with the name "de Pomplune", coming from Pamplona. They lived in relative peace until the Bartholomew's Day Massacre on August 23-24, 1572. The Camino de Santiago shows some of the stages in connection with their fate. These people went to Holland, Germany, where they hoped to live in peace again. In the course of time their name "de Pomplune" changed to Pomplun and some other variations. I think this explains the provenance better than a slavic origin.
Prussia was a good choice at that time,also for Scottish people, Dutch, the Huguenots who had fled from France, etc. A large number of immigrants came there fleeing from the plague. It was protestant and all people got a piece of land from King Friedrich, who wanted to cultivate this part of his kingdom.
Let me know what you think about this possible background.