OP, I am really sorry that you are hurting. From what you said, you and your wife got in to poly as a way to "round out your marriage." Is it possible that she was unhappy with the marriage and used poly as a way to meet someone to replace you? I apologise if that sounds cold or hurts to even think about it. People do cling and hold on to relationships that they sometimes need to let go of, but they choose not to do so until they have someone else lined up.
If your wife comes back, would you really want to take her back?. At five weeks in, she is introducing this person to the family and all that. That is fine for her. My concern is your daughter. Is five weeks even long enough to obtain a background check on someone? I am not a fan of the whole NRE-is-why-I-left-my-spouse-and-broke-apart-my-family crap. No amount of great sex, lust, or so-called love is enough for me to put my children through a painful divorce and custody dispute. I do not think you were being unrealistic in wanting to insure that your child had enough time with mummy and daddy. If mummy was gone x amount of days out of the week and daddy was working and seeing the girlfriend when he was home, when did she have time to see both of you? Children needs should come first.
Are you poly? You may or may not be. I would not worry about that right now. Take some time to heal and wrap your head around all that has happened. Distance yourself from your wife and only deal with her when it comes to your child. Avoid saying anything negative about her in front of the child. Keep your emotions out of it if you have to see her. I wish I had more words of comfort, but nothing I say could possibly make you feel better. I truly am sorry, though.