I think you're being selfish. It's his child and you can't say no to every single option (saying he can stay but only if he does what he's told - his dad can't force him to behave can he?).
I think you're letting your feelings about children make you crazy. I don't see the relevance of your DP WALKING NEXT TO a colleague with a baby (can you genuinely not get past your rage to see that this is objectively nuts?)
You got together with a man who has children. Act like it.
And I think it's fine for her to ask him to look after her daughter - she didn't force him she just asked and he agreed as it's his sons sister a.k.a your DPs family. Adults, living their lives, working towards the greater good for all children involved.
If you do go on to have children, please don't think that they're more important than his other children or that he doesn't need to see them anymore because I can feel you edging that way.
You keep calling everyone sheep - generally when people all agree on something it's because it's the correct or true thing not because they just feel like following each other. It's a cowards defence for when thing aren't going your way. It's what D. Trump does a lot actually.
I was also a step child. I don't have children, I'm only 24, but have a DP if six years who is kayaking with his friends. We're at a house in Wales having a weekend long birthday celebration but I stayed in today as I have an exam next week and am taking a wee Mumsnet break from revision. Neither lonely or sad... just stating an opinion. It's pretty unanimous so I would take a long hard look at yourself and your behaviour if I were you OP.