I think the entire death scene was about defying fate. It felt like it was fate simply because of what she said, but the death cards were literally made for the kids. Out of her own good will, she chose to save those children, which is why the kids' death cards disappeared, and hers appeared after. The whole grim reaper spiel was about how God cannot calculate for a sacrifice a human will make, and that it is the choice of the human to take it, kind of shows that it wasn't fate.
She was free to keep moving, she would have avoided the truck, but she hit the brakes instead. Her fate then and there was to live, but she chose defy it and sacrifice herself. It doesn't state that she felt like she wasn't in control of her actions, she just said "If I move then the kindergarten bus will" and the slammed the breaks down. With the conversation with the grim reaper that you pointed out, she said "Why is it like this? I knew it, but I stayed like that, I was so scared up there." All that says to me is that she had saw it coming, but chose to stay like that.
Eun Tak's death card being late is pretty solid evidence. IIRC there was never a late death card notice that occurred after somebody died. It was always received prior. Even when god was "trying" to kill her before, the death cards were always in advance.
I'm not going to lie they do it really confusingly, going back in forth between fate and choice, but I think for me it was entirely her choice.
- NORTH AMERICAN TOUR 2017
