"Winning a skirmish": In the Napoleonic wars, 2/3rd's of the battle was the pre-fight marching and combining of corps and armies, artillery and cavalry and infintry, so that when the first shots were fired, your combined forces were the larger. Or think about when Presidents Xi and Biden shook hands recently; Xi's advance team made sure Xi was standing center stage and Biden needed to approach him, not vice versa.
Well, an 'all 3 of us together' social event with his ex is similar--the context of the event is everything [and just as treacherous--meaning, the ex must be assumed to be using the occasion to sow mischief].
> Under no circumstances can the seating/standing be such and he and she are sitting adjacent--I must be between them at all times.
> She cannot whisper unheard asides between them.
> Any anecdote requiring a backstory must be fully explained before the story gets to its punchline. [Nothing worse than later, when I'm back alone with him, asking him 'What did she mean when she said xxxxx? (...and I was just standing there with a dumb smile on my pretty face).]
> Stay more sober than her.
> If her face is breaking out, be merciless with that. Dare her to make comparisons between her body and yours.
> No, you do not have any plans to unblock her of social media anytime in the foreseeable future.
> Any changes to financial terms will go thru the attorneys, thank you.
> No, actually I didn't have a better time than I expected.
> No, actually I did find this awkward, or at least uncomfortable, and will contnue to do so.
> Towards the conclusion, when she says 'We MUST do this again,' be non-comittal: 'Maybe,' 'We'll see what happens,' 'He and I will talk about it.'
Again, go back to the scenario of the civil, well-adjusted divorced couple/(s) doing the weekend custody exchange with the kids. It's all-business. There's no long conversations. There's no witty banter. There's no cheese and cracker platter. The minimal required interraction required by the Court.