I am not going to judge your decision to go out with a married man and then getting pregnant asap. Life is not black and white and sometimes, terrible things happen for the best long term.
However, you come across as very self-centered, showing not only little sympathy for a woman who could be you, but even contempt. You are lucky to have now what she thought was hers, but you don't know what tomorrow will be like. She is a mum who has been cheated and left behind by her husband. Of course she is going to be unreasonable to your needs, why should she care that your life is unaffected as possible to suit you? She has enough on her plate to cope with, and if she can hurt you back by some of her actions the way you have hurt hers, then why should she feel bad?
You need to stop thinking that you can have it all. You can't. At the moment, you have a man who has an ex with an axe to grind and you don't deserve to have it easier to cope with. You are with a man who will have a duty towards another child than the one you have together, that includes supporting her financially.
The fact is, maybe she was a horrible woman and he deserved to find someone (like you) to treat him better, but if deceiving her about you wasn't bad enough, he continued to deceive her by lying about your pregnancy. Did you really had no issue with this? Can't you see that the whole thing about avoiding stress is nothing else than an excuse? You didn't have to be stressed by it, he did.
Do you really see nothing wrong with him quiting his job and by doing so preventing him from his responsibilities because it suits your new life together? Even if guilt make you both agree to give a token £100 a month when he would otherwise have to give more, do you really believe that from a point of principle, that's being good caring parents?
How you can think so highly of someone with so little scrupules is beyond me.