Swing I agree and that is why I think it's really important (although hard for some) that ex's try to embrace new partners and include them a bit
Absolutely, but as you've stated yourself, many new partners do not want that and would rather have the Ex has far away from her life as possible, and as you've said, that's understandable, but it can't go both ways.
I could give many examples of the changes my ex made to his relationship with our children to appease her sense of insecurity. It was her who struggle to understand that what he did for the kids had nothing to do with keeping a link with me. This was the outcome of it because they were still so young, but it wasn't the intention at all, not from him, not from me, but that's all she focused on. She wanted him to have no contact at all with me any longer and that inevitably led to him becoming a different dad to the one he had been to our kids.
I know a lot of people in here will say I'm wrong, but there's often an attitude of mum's (and understandably) that the first family (hate that expression) should be the priority and new partners etc should step back and know their place
I really don't think this is it. I think it's about defending that it shouldn't be about priorities one way or the other. It's very similar in my views to going from one child to a second, where the first child has to adapt to a change of attention (which often comes with many tantrums, understandably as it is a difficult adjustment), but it doesn't mean they become less a priority as most parents will still make sure their first feels as loved as before and will try everything to make sure that they don't get less than before.
I guess I have a strong personal view that if you are not in a position to give more of the same (or close too), then you shouldn't expand your horizon. This is why many parents stop at a certain number of children even when they would love to have more, because they know that if they did so, their existing children would suffer.
You mention not making it difficult for the new partner to be involved in things like attending parents evenings etc... I think that this very much depends on the involvement the SM has taken into her SCs life. Being a parent is a balance between a lot of difficult and stressful shit, but then also the reward of affection, pride, love. I don't think you can demand one without being prepared to take on the other.
My view is that if a SM is prepared to act as parent, ie. take on some of the responsibilities, financially, time, tiredness , then yes, she should be entitled to be party to important aspects of their lives like parents' evening. However, if she makes a point that she shouldn't take on any of the above responsibilities because she's not the parent, then she can't expect to be party to activities that are as parental as it gets (hence why you don't normally see grand-parents, uncles etc... joining in unless they do act as parents themselves).
I do agree that some exes would have an issue with another person coming into their kids life as a parent because they feel that this is something that belongs to them and them only. I'll be totally honest that this was my biggest fear as I felt extremely maternal (from a parenting perspective) to my children, so would have hated that some woman (who happens to have a totally different background, principles, attitude to life etc... to me) feels she could act as a parent to my kids, but thankfully, she felt exactly the same and never crossed that line.