As a mere MA in experimental psychology I would still like to put in my 2 cents worth. Borderline, antisocial (psychopathy) and narcissistic personality disorders are almost by definition due to an underlying lack of ability to perform certain brain functions (like feeling empathy). Think of it as the emotional equivalent of dyslexia. Part of the problem with these three personality disorders is that they don’t distress the person having them, they distress the people around them who suffer from their actions.
Learning musar is not going to help a dyslexic learn to read – special techniques are needed. For people with these types of personality disorder, change is very hard, because the person doesn’t think they need to change. Psychotherapy with such patients is very, very hard and seldom very successful. The people who most benefit from therapy are the family members who have to deal with the person.
So no, musar is not going to help the patient. Psychotherapy might, but maybe not. Anyone who thinks they are involved with someone with one of these disorders should seek help for themselves, to deal with the stress of the patient’s behavior, and also ask relevant sheilos as they come up.