"He seriously spends excessive amounts of energy just finding ways to be disruptive and difficult"
Umm no. It comes quite naturally to them.
Everything I have learned so far about setting boundaries is that
you absolutely will get pushback. Mean, controlling narcissists don't
like losing control -ever, and it's the worst when you begin enforcing
boundaries.
So, plan on it. For example, at the bd party, can you get a few big guys,
maybe family or friends to be around? Nothing works better to
intimidate a cowardly narcissist than a show of force. Nothing needs
to be said or done, just have them there, very conspicuously. (In
case he decides to show up)
And for your sweet kids, seems like telling them, "dad's not
following the rules, and these are the consequences" is
enough.