nso birth cerificate conflict because of mother's first marriage

Dear all,

Need your advice for my current situation. In my municipal birth certificate, my surname was under my father's surname so ever since, I've been using that surname to my documents such as diplomas, ids etc. However, when I claimed my NSO birth certificate, my surname is under my mother's surname so we provided affidavit of legitimation and other documents required by our municipal hall in order to process my change of surname to NSO Manila office. After submitting my documents to the NSO Manila, they found out that my mother had a previous marriage. Although she was also married to my biological father. According to my mother, her first husband abandoned her 10 years ago and currently nowhere to found so here's my question:

1. How to null void my mother's first marriage considering that her first husband abandoned her since 10 years ago and currently nowhere to found now? What would be the best option? Annulment or legal separation or affidavit?

2. With regards to my NSO birth certificate, are there any documents needed so i could finally change my surname into my father's surname?

Your advise is really much appreciated. Thank you.

very tough situation.

I would consider a petition for judicial relief to compel the NSO to recognize your birth certificate under your parent's marriage. yes, it appears to be bigamous, but unless there is a court which has declared that marriage as void, it should have a presumption that it is valid.