Great posts SirChing (great name too!), you’ve summed it up brilliantly. When OP said her ex told her he wanted to split up whilst their son was in the room, I was pondering what kind of person does that.
Whether he’s a narc, cunt or bell end, maybe all three, agree with maintaining superficial friendliness, it’s in your interests OP but you’re right to be wary.
It’s incredibly impressive how you’ve dealt with your emotions and got yourself to the point of not having ill feeling. It sounds like you’ve got the ability to be more detached in your assessment of the situation. When someone is still really hurt and angry with how they’ve been treated, it’s harder for them to be objective but your spidey sense and instincts seem to be spot on. Don’t trust your ex’s intentions but don’t alienate him either.
It’s clear you have your son’s best interests totally at heart but you may also have to accept that if Wednesday is your son’s day with his father then it’s for your ex to decide how he manages it and he doesn’t need your permission. He’s involving you which is positive but if you start to become (in his eyes) obstructive in relation to his days, your ex will stop involving you.
Re half term, maybe encourage your son seeing ex’s mum but don’t show you object to the GF taking him out. Ultimately unless you have real concerns about his safety and well being, you may need to accept that your son will end up being taken out with the girlfriend if not at half term at other times. Developing an independent relationship with her may be in your interests in the long run. Your ex may think he’s “won” in creating a blended family but your motivation is your son’s well being, your self preservation and maintaining control, not your ex’s happiness. So even if the outcome is the same outwardly, inwardly you can reconcile your actions.
You’re amazing OP as others have said. Really hope you can find someone to move on with too. Wonder how happy your ex will be to blend with another possible father figure. Sowing the seeds now for how you would want a future partner treated would also be a good outcome for you and your son in the long run too.