WAIT I THINK I DISLIKED YOUR COMMENT I DIDINT MEAN TO MY FAT FINGER GOT IN THE WAY
But she died already
Yes, but she died for a reason, the possibilities is that either the reason for her death will change making "meaningless" her death, or that the world would be rewritten in a way in which Koichi died after his mother, so, his mother sacrificed was for "nothing".
Koichi was expecting that since he never exited in the world, his mother or father could still be alive, giving a purpose to his absence.
I can honestly understand those feelings, either way it feels awful, I don't blame him for wanting to come back.
I would personally stay, I'm selfish enough to rationalize it in "this is what my mother would have wanted *cries*" kind of way.
It might seem like a stupid decision because either way his mom gonna die. But personally, I think if you love someone so dearly, which Kouichi is towards his mother, i would wish to be with them until the end... What's more he's still young and his mother is a very important presence in all his life, if she's going to die, he wanted to be with her until the last of her