This one has stuck in my mind over the last couple of days. Having read all your updates OP, I think that to a significant extent, what you are doing is over-compensating, in an equal and opposite way, for your own (former) doormat tendencies, by raising your dd to be selfish.
You're focusing too much on your and her internal experience, too little on empathy for others and developing the skills required to interact with people.
What you both seem to be missing, is that assertiveness is based in good manners. If you know how to talk to people clearly, nicely and in a way that connects with them, then including your own needs in what you say comes easily. Haven't you noticed how the most socially polished people are exceptionally to the point and no-nonsense about stating what they want and making it happen? All without upsetting anybody?
Good manners are fundamentally about consideration for other people. Learning to look outwards, read people, think about their needs and communicate accordingly. Not stewing incoherently inside, until you're in danger of exploding.
So, while I get what you're trying to do, I think you're stuck in a rather passive, inward-looking mindset, that sees other people as a threat, rather than recognising them as fellow feeling, thinking people, with needs, preferences and struggles of their own. You need to teach your dd to perceive and connect with other people better.
So in this instance, I think you got it a bit wrong and that that would have been true, even if child A had seemed ok with being ignored.