Changing the contents of koseki tohon

Hi, I have a question and I’m hoping someone can answer.

For context, I’m a Japanese citizen and my now partner is a filipino citizen. She was divorced a few years ago to her ex-husband who is also a Japanese citizen. Her surname is still her ex-husband’s even after they got divorced. Apparently they don’t have a divorce in the Philippines and the only way she can change her surname to her maiden name and after that to my name after we got married is through a process called “Recognition of Foreign Divorce” which you will appeal in court. It’s a very long and time consuming process which involve a lot of money because you will need a lawyer for that. We are now in the process of that.
A few weeks ago, we found out that she is pregnant. As we are not still married because we’re still waiting for the result of the ROFD, we went to city hall and applied for Taiji Ninchi(recognition of unborn child). We were told that we should just apply for a marriage registration as it will be easier that way. They just need some papers which can be produced quite easily. We can be married in Japan but we cannot register our marriage in the Philippines as she is still considered married to her ex-husband in the Philippines. after we get married, her surname right now will be registered in my koseki. The question is, after we have finished all the legal work in the Philippines: ROFD, registration of marriage, name change etc. can her surname be changed in my koseki to my surname? Also i think the child will be born before we can change her name to mine thus the child’s mother’s surname will be her ex-husband. Can i also change that?
I hope someone can answer this.

Hmm, no expert here. But if you are allowed to legally marry in Japan. (Don’t you need some kind of document that says you are both unmarried???) anyway, if you can marry here isn’t the law in Japan that one partner must change the surname. I know it doesn’t necessarily need to be followed for couples where one partner is Japanese. I think it was mainly to avoid that Japanese women marrying a foreigner end up with a none Japanese name. But if she just says that she wants to change her surname at marriage what would stop her.
Best to ask however to the office that allows you to marry.