that have had such fluid borders and blinked in and out of existence so frequently?
Usually in history once a country blinked out of existence, it never came back again. Poland is among the exceptions.
BTW - "so frequently" is when exactly? Poland "blinked out of existence" once, in 1795. Of course we can argue that it came back as the Duchy of Warsaw, transformed into the Congress Kingdom of Poland, then blinked out again in 1864, and then returned in 1918. This is still not "frequently".
1939 - 1945 was illegal military occupation, which according to international law is not equal to "blinking out of existence".
Chopin, Copernicus, Curie, Luxembourg, Conrad, et al.
Curie was Skłodowska, but she married a French guy and adopted his surname. Conrad = Korzeniowski, Conrad was just his nickname or pen name.
Copernicus is a Latinized form of surname Kopernik (which is still a frequent surname in Poland today).
Róża Luxemburg was rather famous as a devoted International Communist (despite her Polish ethnicity), not as a person of merit for Poland.
German-sounding surnames are frequent in Poland. My mother is de domo Meller, for example. Though it is Dutch rather than German.
How many countries do you know that have had such fluid borders
England, France, Germany, Russia, Greece, Bulgaria, Turkey, Serbia, Denmark, Spain, China... and many more.
Britain has perhaps the most fluid borders, for example British borders once included India.
Polish borders contrary to myths were quite stable.
There were plenty of minor or major but temporary changes. However, major and long-lasting changes were relatively few.
If not counting not-fully independent "Polands" of the Partitions Perod (1795 - 1918), then distinct shapes of Poland were 5 in history: 1) since the beginning until the 1300s; 2) since the 1300s-1400s until 1569; 3) since 1569 until the Partitions; 4) since 1918-1922 until WW2; 5) since 1945 until present.
These are the five "recognizable shapes" of Poland.
Minor - or major but temporary - border changes were of course more numerous (all in all ca. 180 border shifts during 1000 years).
Modern Polish borders are pretty similar to those of the early Polish realm until the 1300s.