I do find strange as why did he make a big song and dance about “you should be happy she is leaving the team” if 1) he didn’t think it’s an issue anymore and 2) they want her back anyway.
It does make sense. Because there are 2 sides. When at work I am in work mode. And if, for work reasons, we wanted someone back I would be happy with this. My personal feelings to their actions, years ago wouldn't come into it. Especially of they were acting how they did 2 years ago.
My exh hated my last boss. Because we got on. Nothing inappropriate ever happened between me and my boss. While I felt bad that exh, felt insecure about that. I had a job to do, for many reasons I was better off working that boss. That's boss has now moved office. But if I got the chance to work for him again, I would jump at it. Because it's good for my career.
If you have recently had a baby, then that's probably contributing. But going over him staying out late etc doesn't help and isn't relevant. You are making up the situation that he was staying out to see her.
There was nothing but work chat in those emails and you are blowing it up into something else. If he started texting and trying to reassure you all day, he wouldn't then be doing his actual job and you would probably start reading more into that. Because you feel vulnerable and just had a baby.
You need to remember that you didn't find anything bad. You need to take a deep breath and calm down.