Sorry, but unless you have the money that it takes to litigate this, nothing will change.
You don't get to adopt without the consent of both parents.
You don't get to do a name change without the consent of both parents.
You'll just have to settle for being guardians without changing the child's last name.
By the way, the reality is that the name doesn't define the child and I think, like most, you want the child to carry your name because of your own emotional needs, and not the child's.

