Have you tried to see it from her pov.
She got into a relationship with a man who was separated. There is no evidence whatsoever that he told her that he was trying to work his marriage out or planning to have a child with you: as far as she was concerned this was a relationship with nothing wrong about it. What do you think she should have been old enough to know better- not to get involved with a man with an ex?
Then all of a sudden she is told that he has been back and got you pregnant. Now from her pov she is actually the betrayed person here and you are the OW, sleeping with someone (your ex) who is already in a relationship with her.
He does not tell her that he is breaking off or planning to break off with her: to the contrary, he tells her that he is just supporting you through the pregnancy, i.e. not in a relationship with you. Again, from her pov the current relationship is the one he has with her. At the same time she is getting worried because he is clearly behaving oddly, hanging around you and not supporting her. Hence the frequent texting. Wouldn't you? If a man who had promised you a relationship was suddenly living with someone else?
The fact that you had a newborn doesn't make it less of a betrayal of her. What would you have said if he had had a baby with another woman while you were still married and gone off and spent time with her- would that make you the OW just because she had a newborn and needed support?
Conclusion: your ex is an arse. This woman is no more the OW than you are. She entered a relationship in good faith and has been betrayed. Just like you have.
She is trying, albeit clumsily, to behave in a civilised manner and be as positive towards this new baby as she possibly can for its own sake, though she can hardly be delighted with its conception at a time when she had good reason to believe her father was in a relationship with her. Of course you don't have to accept her suggestions. But do try to see that there could be more than one angle here.