Changes look good, I'll be starting a new game soon and will look more thoroughly then.
The thing with Polish capital constantly changing is kinda weird. Looking at wikipedia, it seems that the first Polish capital was Gniezno. Warsaw indeed didnt exist at all this time, and Krakow was one of the land gains of Mieszko I. If we are going for adopting history, we should go for Gniezno or Krakow (Gniezno did not last much). Krakow was the Polish capital for long, while Warsaw was for a long time capital of Mazovia.
Forcing Gniezno would be like forcing Winchester for England and Guimaraes for Portugal. Ideally I'd go with Poznan as it also lasted as a major city. Polish historians still argue whether the title should go to Gniezno, the coronation site, Poznan, the largest city and actual center of administration, or one of the pre-Christianization sites...
Capitals of Poland:
Giecz - regarded as the capital of Piasts in 10th c., before Christianization
Gniezno - regarded as the first capital, but in reality it was much like Aachen or Rheims - a ceremonial site
Poznan (alongside Gniezno) - the most important city, first cathedral, seat and burial site of kings
Krakow - moved here in 11th c.
Plock - moved again in 11th c.; just slightly west of Warsaw, I suggested this name but given the mod's timeline it's a bit too shortlived
Krakow - 12-16th c.
Warszawa - from 16th c.
The first three - Giecz, Gniezno and Poznan are located in the same area, left of Warsaw, above Wroclaw. If you want to be historically correct and use this location, you'd have to move Wroclaw further south or something, because it's too close. Anyway, you have a list to choose from in case Warsaw is out, but after playing till 16th c. personally I don't mind it anymore.
There's a some imbalance in food, however. Warsaw skyrockets early and can easily sustain pop of 25+ while Krakow grows very slowly and can sustain from 10 to 14 (if you build windmills & take over all the shared tiles) with nothing but iron to make up for it. Pigs or apples could be moved from Warsaw to Krakow to balance it.