Does one who unintentionally reviles a Qur'anic verse due to difficulty in memorization become an apostate (kafir), and is there anything they must do to atone for that?

If you did not intend to revile anything from the Qur’an, and that occurred by mistake, then you are neither sinful nor an unbeliever. For Allah has pardoned this nation for mistakes and forgetfulness. Allah Almighty said: "Our Lord, do not impose blame upon us if we have forgotten or erred" (Al-Baqarah: 286), and in His response: "I have done so." And Allah Almighty said: "And there is no blame upon you for that in which you have erred, but [there is blame for] what your hearts intended. And ever is Allah Forgiving and Merciful" (Al-Ahzab: 5).