Is it permissible for parents to reject their daughter's suitor despite her approval and desire to marry him? And is a girl's talking back to her parents and yelling at them considered disobedience (Aqoq), knowing that they mistreat her?

If the parents are abusive and harsh, it is not permissible to treat them in kind. Rather, one must be patient with them and treat them kindly, even if they are disbelievers. You must repent from yelling at them and seek their forgiveness, while frequently supplicating for their guidance. If you do not have money, your financial support is obligatory upon your father until you get married. If a righteous man proposes to you and you desire him, your guardian has no right to prevent you from marrying him, even if the closeness in age between the spouses is a consideration. You can seek assistance from some people of virtue or trustworthy female friends to advise them.