Why is "This chapter is long and difficult" not correct?
This chapter is long and difficult - הפרק הזה ארוך וקשה, זה הפרק ארוך וקשה. Long and difficult here are adjectives describing פרק, and זה is a pronoun and the subject of this sentence.
Native English speakers would say "This is a long, hard chapter" not "long and hard".
Eh, I could see it either way: "It was a long and difficult chapter of his life". And I would probably only say "long, hard" if it was innuendo.
ze pérek aróch ve-kashé.
is "פרק" both a book chapter and a series episode?
Is it קשֶה (qashe) for male and קשָה (qasha) for female?
Yes, and the words are spelled the same and only differentiated with niqud.