How can one atone for not asking the father for forgiveness before his death, knowing the psychological circumstances the son was going through?

If your argumentation with your father is within the bounds of etiquette and intended for advice and reform, then there is no harm. However, if it oversteps etiquette and harms him, then it is an act of disobedience for which repentance is due. You must frequently supplicate for your father and show him righteousness in whatever way you can. Waswas (obsessive whispering) does not remove accountability unless it leads its possessor to a state of unconsciousness. But with choice, one remains accountable and responsible for their actions.