I know it 's emotional and a tough time right now with Guatemala adoptions. I am sorry you are feeling this way right now - not having this child home right now really hurts.
However, I have to say, what you are experiencing is not "abnormal". Many foster moms are instructed to not call the child by their new name for several reasons. One reason is that the adoption is not final until the very end. Things can happen - we all know that things happen with Guatemalan adoptions. Therefore, I know that some hogar's and attorneys do not allow for the children to be called by the new name that they MAY have after the adoption is final. Not a fun thought - but it is reality.
Secondly, with the momma thing. My kids all said momma by the time they came home. Sure it hurts to see them calling for their foster mom - i am not sure they really "know" what momma means - but what it does show is that this child has bonded. This child has been loved and cared for and has bonded with their foster mom. That is a much better situation than a child who hasn't bonded or is unable to bond. So, although it hurts you to see that bond - the fact that this child HAS bonded is a great thing! Trust those of us who have gone through or are going through working with a child with bonding issues.
Hope that makes sense!
One option that you may want to think of has already been mentioned - fostering yourself. That would be a great alternative for your situation if it is something you can do!
Take care!