Sadly there is not really a one-size-fits-all solution to battle parental alienation. Rather than treating it as a legal or moral challenge with legal and/or moral solutions, consider instead adopting psychological strategies that pressure the narcopath into behaving. In short, “impose by consequence”. Incidentally, such psychological strategies are also the best way to resolve divorce conflicts, settlements and any other issue.
To do this you need to understand NPD to a deep level, understand their Achilles heels, their motivations and drivers, what it is that fuels them and what it is that wears them out. Who are their allies, and where do they derive their power base? How can you put uncomfortable pressure on them legally?
If you want an ally in the battle, someone who understands the narcopath very well, and who uses their army-office training to develop strategies with military planning and precision, this is precisely the sort of work that I do. I will happily give anyone 15 mins of my time for free.