Ok focusing on his behaviour... There are a few elements to this: Your curiosity, his level of brain development, his emotional intelligence and your iron clad boundaries. You say what you mean and you allays follow though. Be predictable. It feels safe and containing.
What do you think is motivating his poor behaviour in any one given moment. You've not given a massive amount of detail in your examples here. But you wrote:
"Standing in front of other children and then complaining they had knocked into him, just a litany of ‘X pushed me! X jumped on me! X hit me!’ and X hadn’t (and I was watching both ds and X like a hawk!) But he was putting himself in positions where X would run into him or push past him"
So what do you think was motivating this? You know him best. I have suggestions, do any sound plausible...? Maybe he simply doesn't yet know how to play with other children cooperativly. There are stages to learning how to interact with others, maybe he's still at the parallel play stage. It's a brain development thing, just like fine motor skills needed for handwriting. To help, you and your family could role-play social play with him at home, developing turn taking skills, listening to others ideas and deferring to those sometimes and also taking the lead other times.
Or maybe he was angry at X for something that happened earlier, which you might not know about, but he wanted to get X in trouble to ease his sense of injustice. If that rings true, the best response in the moment would be to say "You seem like you're feeling angry with X, and want me to tell him off, why is that?" Narrating any behaviour you don't understand is a great tool. Just say out loud whats happening and then wonder out loud why it's happening. If you have a theory of why the behaviour is happening suggest it, see what they say. Wondering about his emotions in any given moment will help his emotional intelligence. Help him recognise his own feelings and those of other people too. Eg "you look very angry, your fists are clenched and your face is scrunched up. Do you feel angry right now" The trick to this is staying calm and curious yourself. Don't allow your worries about how other parents feel about your parenting to get in your own way.
Or maybe he wanted your attention. He wanted you to see him in his world. He was one kid in a crowd of others, but he wanted to be seen. But had no idea how to go about that in a positive way.
When it comes to your boundaries, they don't feel iron clad. Which is unnerving for kids. Where's the line? How far can they push in this situation? Or on this particular day? It sounds like you're letting your embarrassment and your fear of being judged by other parents get in your way of doing what you need to do. In this example, you needed to leave the party in spite if your children's dissapointment and a potential noisey (but time limited) scene they'd cause. Your feelings are very normal, no judgement about that at all from me, but if you were able to recognise that feeling of embarrassment in the moment and dismiss it as unimportant in the context of supporting your child's development, it would help you parent him immensely.