The problem with proving narcissistic personality disorder in court is the time, expense, and difficulty to actually prove it.
The effort will be exhausting and expensive and, in the end, could prove futile. The family court system simply isn’t set up to deal with the sh*t-storm associated with narcissism or other personality disorders, for that matter.
Highly-skilled NPD charmers easily dupe the courts, like they do most people. A judge may not see that your NPD co-parent is simply using the court to torture and control you through the kids.
I’ve known a few brave souls to attempt to prove to a judge that their ex is indeed a raging narcissist. None could. In fact, one brave woman tried for over four years – court dates, mandated evaluations, even lobbying government – but nothing came of it.