Is saying to one's parents, "You are crazy; you fight each other like this," considered an act of disobedience (ʿuqūq) in light of their violent quarrel, and how can one atone for that and rectify the mistake?

If you uttered an offensive phrase to your parents unintentionally, then there is nothing upon you. However, if you were aware, then it is an impermissible offense, for the rights of parents are great and are not forfeited by their injustice or transgression.

If they were severely hurt, then it is a forbidden act of disobedience, and its expiation is repentance, seeking forgiveness, and asking for their pardon. You must strive to be dutiful to them by advising them, enjoining them to do good, and forbidding them from evil with gentleness and good manners.

To reform your father and help him repent and become righteous, seek Allah's help and involve some righteous people to advise him and encourage him to associate with the righteous and attend gatherings of knowledge, along with frequent supplication for his guidance.