Doubt it. Tomoe had said she didn't know who the father was and when Ayu was born she only guessed that it was a Japanese man she met out of the country. By saying that she doesn't know who the father is means she slept with multiple men. So if it's only a guess, I doubt she'd let anyone know they're a father if she's unsure that's really their kid. So Ayu's father seems pretty irrelevant to the story. Just my two cents though
I don't know if this counts as a spoiler but...
Your theory is not possible. That's cuz...
Spoiler
Spoiler
Spoiler
Spoiler
Spoiler
Spoiler
Spoiler
Spoiler
Spoiler
Spoiler
Spoiler
Spoiler
Spoiler
Spoiler
Spoiler
Spoiler
Spoiler
Spoiler
Spoiler
Spoiler
Spoiler
Spoiler
Spoiler
Spoiler
Spoiler
Spoiler
Spoiler
Spoiler
Spoiler
The bio dad is already dead. He passed away after Ayu's birth or something apparently. I might be wrong since I'm just reiterating what someone else revealed.
I feel like the mom is gonna die and at the funeral the bio dad will be there and try to take the girls away it's going to be a big heartbreaking moment