I'm looking at it from the possible perspective of the mother, Ladydeedy, something you are seemingly completely incapable of doing. I do not know why you cannot even get close to seeing things from the mother's pov. She is not just some nuisance you can swat away, some obstacle to be rolled over by the tank of your logic and superior appreciation of what's best for everyone.
What exactly I am saying here is that you are not the child's mother. Like it or lump it, you simply are not.
I can guarantee that you would see things completely differently if you had a child of your own and circumstances like this arose. Logic would be the first thing to fly out the window once you beheld your child for the first time, and outside the window it would stay. You would object so very strongly because your child is your own flesh and blood, and that is what makes all the difference.
You can bleat 'shame on you' however much you want. You would be better off, and so would the child, and possibly the mother, if you were to expend some of your considerable energy trying to see things from the mother's point of view and working with those insights you gain. The attitude you have, that a stepmother and a mother are fungible, will obviously get you nowhere with this mother. Even from the pov of going through the courts quickly and efficiently and getting this thing settled, it is misguided. This woman clearly needs her primary role and relationship in her son's life acknowledged. Instead you seem hell bent on dismissing her.
Can you achieve what you want without going for the nuclear option (the PR)? Because that is what this is. You are using a sledgehammer to deal with something that requires a much more delicate instrument. The best step-parent-biological parent-child relationships I have seen are those where everyone takes pains to acknowledge the primacy of biology over legal definitions.
Whether the mother has shared information with you or not, you have no right to post it here, or share it with people in RL, unless she has given her permission, just as she should not post your medical details if she knew them.