How would 2 God-Parents be able to get custody of our God-Child?

You won't like my answer. You have no legal rights to the infant, anymor ethan any other person.
What the mother can do, is leave her preferences in a will. The courts are not obligated to follow her wishes. Unless there is a stepfather in the future that is willing to adopt the child legally, the courts will not terminate a natural father's rights, unless there are extreme circumstances.
Since the child is unborn at this time, in most ststes , the mother must wait until birth to bring an action for custody and child support. She cannot waive child support. The father has a right to visitation, unless he is a danger to the child, proved in a court of law, and then he still may get supervised visits. The mother needs to retain a skilled family law attorney to head things off at the pass; have the petitions ready to go upon birth. There may be a legal stragedy the mother can use to keep you in the legal picture though. She would have to confer with an attorney. It is possible to inlcude third party visitation, by court order, visitation for you. What this would do is 1) establish for you, some legal standing 2) promote a relationship with the child that woudl later be important to the "best interest of the child" in future proceedings ( maintaining a relationship would be important) 3) possibly perserve your visitation if anything did happen to the mother, and the father gets custody.
As I said, she would have to discuss this with her lawyer. She might want to consider, creating a a durable power of attorney from her to you, to be used ONLY if she is aunable to make decisions, so there will be an "instant" guardianship for the child. Of course the father, if his rights are intact, will be able to overscome this in court, but it will give you standing to even be in court on behalf of th child. Otherwise, you are not a legal party to the case.

The mother can make a will and state her wishes for whom she wants to care for her child,however even with a will,if there is a fater out there,then the father will be the one to be able to get the child,even a family member would have a better chance than a non-relative. I think God parents is great,but being God parents does not mean that you can get custody,or the mother can leave the child in your care. If the father finds that the mother was ever deceased then he would be able to file for custody. Being unstable can mean alot of different things,will he always be? My sujestion is for the mother to leave a will,and to talk to all family members and let them aware of her wishes,so that they will not interfere if the father decides he does not want to raise the child,and the father can give you custody,if anything happened if he wants to. But you know all though I understand where your coming from, I am sure this may never occur,and the mother will live to raise her child,being perpared for the future is a good thing,but we really don't forsee these things as happening.