If I am correct, Kievan Rus was set up, in the sense of an organized state, earlier than the Poland. Kievan Rus is 882 while Poland went from tribes to statehood in 966. Thus, my theory right now is that people from Kievan Rus went west and set up Kujawia in Poland. The name is simply an honor of Kievan Rus's legendary founder Kiy and the city of Kiev. Settlers continued to follow this trend all the way to the end of the colonial era – many cities in the US are taken from older European cities. The spelling discrepancy between Kuyavia and Kiev could be attributed to the lack of language standardization at the time.
But as you say, concrete research would be most interesting here. A way to end that lethargy.