My friend, you need to listen to those signals. You're smart enough and observant enough to have acquired the necessary data, but you're so into this woman that you're squashing the alarms.

You. Are. The. Backup. She is actively looking for a relationship with this guy. You can walk away, or you can issue an ultimatum, but you are not her primary dream right now. Her dream is that she gets together with the man who got her pregnant and they live a happy life together. You are the just-in-case, and she probably likes you a lot, but you are not her priority. If you're ok with that, great. If not, break it off. Call it "giving her space" because she's clearly needing to sort out some things, or whatever. But the current situation doesn't sound like it's good for you, and it's not just going to go away while you allow your girlfriend to test the waters with her baby daddy.