Is mistrzostwa genitive? I'd rather have expected accusative here...
It's accusative plural.
https://en.wiktionary.org/wiki/mistrzostwo#Declension
https://forum.duolingo.com/comment/14878229?comment_id=44653766
Why does the English source use the singular, if the target meaning is plural?
The Polish word is grammatically plural, but the meaning is still singular, one tournament. Which I believe is singular "championship" in English.