I did not read in your post as to whether the mom and child will be allowed to continue contact? As an older child who does and will remember "mom", the devistation of separation and loss would be a path best avoided, in my opinion.
Our family has two situations similar to this. One is myself. My grandmother gave me up to provide a better life for me after Momma died. She went through many years of questioning that decision, but always kept her phone number listed in the phone book in hopes I would contact her. I was just about six when I was given up. When I was 15, I called her out of the blue. I had been adopted for 5 years. We stayed in touch into my young adulthood through letters and phone calls. My main question? Was I really loved by my birth family? I had to wait for many years to hear the answer from a family member.
Next is my own son, his ex-girlfriend lost custody and parental rights to her two babies, ages 1 and 2. They were in foster care until my son became stablized(sober). He has them now, but I have been in contact with thier mother recently. All she wants to know is if they are okay. She has dreams about them alot. She feels devistated by her actions which resulted in the parental loss. Our contact has resulted in my son's anger towards me. Neither one of them were mature enough to handle the children and both were "guilty" of being selfish with alcohol abuse.
My point is that with your new child, there are more avenues to consider than just her well-being. As she matures, she may question her circumstances and while it may not really be anyone's "fault" that the situation developed, she may blame those closest to her. At each maturity level, such as going to school, change of life, dating, etc, she will experience a mini-cycle of grief and loss issues, question and answer, and self-modification. It sounds like you are aware of these issues and are more than willing to address each one as they are needed. And you are right, getting the right support is paramount. She may or may not have issues that you will see need to be addressed. Chances are though, you will.
I do have alot to say on these issues and I would be more than happy to help if I can. Please keep me in mind for the future, if you would like. For now, I would recommend some type of mediation, a go-between to keep daughter and mother somewhat informed as to progress. The love and investment for all concerned may give this child a step up on the ladder of adjustment and attachment.
This child will always want to have their parent in their life,irregardless of adoption laws.I would suggest being a friend as well as a parent. The first post was a very good one. I would hold onto those thoughts.