This is quite a creative idea. I've seen Saishuuheiki Kanojo and I think writing a fanfic based on it would be, well, difficult; both conceptually and emotionally.

However, you've done rather good job so far of capturing the over all "flavor" of Saikano. As for the crossover: it works. And it works quite well. The bleak, war-torn world and the sense of dying hope is present is both series, so they mesh together very well.

I would love to see this continued.

Hm.

While I'm not familiar with the anime you are writing with, I feel you did a good job at expression the emotions and feelings of Chise and Shuu. I enjoyed the brief scenes with them.