See clip from Attorney Rajiv S. Khanna's conference call video that addresses this question. https://www.youtube.com/watch?v=ujMQ79pgzX8 FAQ Transcript Here is the situation. What is Cross-Chargeability? Let me explain. Normally when we do analysis which country quota do you belong to for your employment based Green Cards we go by your country of birth. If you are born in India and you are citizen of Japan, you are still going to be charge to India not Japan. If however, your wife is born in third country in Japan you are born in India but your wife is born in Japan, you can be charge to Japan that is called Cross Charge-ability. That is very useful thing to have, because all of sudden from a heavily backed up country you go to country where dates are current.
So the problem is she is born in Nepal but does not have the birth certificate. Before I go to the birth certificate question, let me go one by one. First is, does USCIS have discretion to deny such cases? Or do they have to give me the Cross charge-ability? The answer is they have to give the cross chargeability. This is not the question of discretion; they are not doing any favors. Once you meet the requirements and you can prove it you are entitled to your cross charge-ability. You cannot be denied your I-485 for this reason.
Now, she does not have the birth certificate from Nepal. Birth certificate is the only way to prove the location of birth, it gets tricky. In normal circumstances if you don't have a birth certificate what you will do is you will get a certificate of non availability from the village Panchayat or Municipal corporation where she is born saying that her birth is never recorded. Then you will get two affidavits from people like her parents, who say that we know that she is born on this date, this place. In areas of cross charge-ability USCIS may require further evidence. It can be any evidence about where she lived in Nepal, things like that but if you try to register her birth now that's not going to happen.
So if you do go through the cross charge-ability and your dates are “current” typically your Green Card should be done within a year.