100% with you and feel this daily (she’s much worse than the examples you’ve given about your partner’s ex though - not invalidating how utterly audacious and insensitive your partner’s ex must be, just providing clarity!) Am 7 years in and ex is still just as crazy and delusional, if not more, and I can’t even tell you how unbelievable the things we have to deal with are. Feel so, SO bad for my stepchild.
PLEASE ask yourself (and be completely honest with yourself) if it’s worth it for the rest of your life, because she’s probably going to get even worse when/if you and partner have kids, get married, or other big milestones in your relationship, and it can totally take a toll on your mental health or sense of sanity.
I wanted to leave many times but found that it was worth it as 1) I absolutely adore my husband, his kid, and our own kid, 2) honestly, I just could never leave my stepchild as he sees me as his “better” mum, and I know leaving would cause too much damage, and 3) husband and I learned to not let it completely control our lives and moods which was a game changer for our relationship and home-dynamic. We just get on with it now, always expecting the worst and hoping for the best.
Hope this helps in any way and good luck OP! It’s lovely to hear you get on so well with his kids <3