In all honesty ... I would do a few things:

- Document EVERYTHING
- STOP talking to her entirely, until it can be documented. ENTIRELY
- Get a Lawyer now. It is cheaper in the long run.
- Have your current child DNA tested. Sad but you have to, who knows what she did in the past.
- DON'T TELL ANYONE, The less you say, the less you have to answer for later.

And to be honest, I'd actually probably knock that guy the fuck out. But I would likely wait til months after the situation has settled. Or have one of my friends do it lol.