IMO, this should be accepted. Report it next time.
For some the verb "to shut" should be translated as "zatrzasnąć", but "zatrzasnąć" actually mean "to slam".
You mean that "i" cannot be heard? Yeah, I agree. I disabled the audio exercises.
You can't. Sometimes, in theory, you could count them, but pluralia tantum nouns take collective numerals, and well... it feels strange to most (?) Polish people to say things like "dwoje drzwi", which is technically the way to say "two doors".
„Dwoje drzwi” seems perfectly normal to me. I mean, you don't need to say that often, but sounds normal.
"Okna" can be a singular genitive, plural nominative, or plural accusative. The object of "zamknąć" takes accusative, so "okna" here has to be plural accusative.
So in order to translate this sentence correctly, you have to know that genitive is not the correct case to use here.