Registration in Koseki Tohon

Hello!

I am a 20-year-old child of a Filipino mother and a Japanese father. My parents were married in the Philippines and their marriage was also registered in Japan. My mother is also in my father's koseki tohon; I am not. Unfortunately, my father died before he even got to register my brother and me in his koseki tohon. That was 18 years ago.

I've never been to Japan since I was only 2 years old when my father died. But my mom and brother did. My mother has also been receiving my deceased father's pension through remittance since his death. My mother never bothered to stay in Japan and decided to just settle in the Philippines and raise us here. However, now that I am older, I am starting to want to know more about my roots and wonder if I could still be a Japanese national.

1. Can my mother register me in my deceased father's koseki tohon?
2. Is there any way that I can get a Japanese passport or citizenship?

As far as I know, I believe the birth report needs to have been made within 3 months of your birth (for a child born outside Japan to a non-Japanese parent and a Japanese parent) and also for you to be 18 years or younger, for you to be granted Japanese citizenship.
By now there may be ways to get “long term resident” visa based on your father’s nationality but I’m not familiar with the conditions.