You told in this
I was the one who always wanted to have a family. Before we married, she always said "some day I want kids" Eventually, after 8 plus years of living together, we got married. Then we hit our mid 30's. We were running out of time...I convinced my wife we should get pregnant... She had her reservations... We got off the pill and 2 months later we were pregnant.
My wife had the hardest time being mom... It was a disaster... Major PPD...I knew our first child would be our only.
Many women have that deep feeling motherly type instinct...My wife never had that and she knew that. She did it for me...I love her for that.
My thoughts considering this quote and the thread here: If she has a history of doing such big sacrifices for you, it might very well be that this open marriage arrangement is another one of those.
You are saying that she is not very good at expressing her emotions to you. Now I ask you: Are you willing and able to hear, comprehend and accept her emotions? In this child issue, you did not really listen to her reservations and talked her into having a baby "for you". Now you say you love her for that.
Now, this opening up thing. Maybe it will be another sacrifice, and she counts on you not really ever taking her emotions for real and that you will love her if she makes herself miserable - just like she did with becoming a mom.
Of course I hope this is not the case, but this history would make me very cautious to proceed.