You seem to be very sceptical that someone with a Narcissistic Personality Disorder can be treated successfully.
The Narcissistic Personality Disorder has been recognised as a distinct mental health diagnosis a little more than two decades ago. There are few who can honestly claim expertise or even in-depth understanding of this complex condition.
No one knows whether therapy works. What is known is that therapists find narcissists repulsive, overbearing and unnerving. It is also known that narcissists try to co-opt, idolize, or humiliate therapist.
But what if narcissist really wants to improve? Even if complete healing is out of question - behaviour modification is not.
To a narcissist, I would recommend a functional approach, along following lines:
Know and accept thyself. This is who you are. You have good traits and bad traits and you are a narcissist. These are facts. Narcissism is an adaptive mechanism. It is dysfunctional now, but, once, it saved you from a lot more dysfunction or even non-function. Make a list: what does it mean to be a narcissist in your specific case? What are your typical behaviour patterns? Which types of conduct do you find to be counterproductive, irritating, self-defeating or self-destructive? Which are productive, constructive and should be enhanced despite their pathological origin? Decide to suppress first type of behaviours and to promote second. Construct lists of self-punishments, negative feedback and negative reinforcements. Impose them upon yourself when you have behaved negatively. Make a list of prizes, little indulgences, positive feedbacks and positive reinforcements. Use them to reward yourself when you adopted a behaviour of second kind. Keep doing this with express intent of conditioning yourself. Be objective, predictable and just in administration of both punishments and awards, positive and negative reinforcements and feedback. Learn to trust your "inner court". Constrain sadistic, immature and ideal parts of your personality by applying a uniform codex, a set of immutable and invariably applied rules. Once sufficiently conditioned, monitor yourself incessantly. Narcissism is sneaky and it possesses all your resources because it is you. Your disorder is intelligent because you are. Beware and never lose control. With time this onerous regime will become a second habit and supplant narcissistic (pathological) superstructure. You might have noticed that all above can be amply summed by suggesting to you to become your own parent. This is what parents do and process is called "education" or "socialisation". Re-parent yourself. Be your own parent. If therapy is helpful or needed, go ahead.
The heart of beast is inability of narcissist to distinguish true from false, appearances from reality, posing from being, Narcissistic Supply from genuine relationships, and compulsive drives from true interests and avocations. Narcissism is about deceit. It blurs distinction between authentic actions, true motives, real desires, and original emotions – and their malignant forms.
Narcissists are no longer capable of knowing themselves. Terrified by their internal apparitions, paralysed by their lack of authenticity, suppressed by weight of their repressed emotions – they occupy a hall of mirrors. Edvard Munch-like, their elongated figures stare at them, on verge of scream, yet somehow, soundless.
The narcissist's childlike, curious, vibrant, and optimistic True Self is dead. His False Self is, well, false. How can anyone on a permanent diet of echoes and reflections ever acquaint himself with reality? How can narcissist ever love – he, whose essence is to devour meaningful others?