I kinda always thought that the core you crushed to disable the enemy robot, actually was actually their life force or whatever. So, maybe destroying that orb, is destroying yourself. That's what I thought could've been a possibility instead of just the robot running on the lifeforce of a human to move.

I'm not quite sure what you mean about the little girl as the replacement. Are you talking about the little sister of the abusive brother, who did not input her data into the computer (or whatever)? Weren't they going to use Chizu (which I thought you were talking about at first)? I thought that since they lost Kako, and since we saw the spinning chairs in preparation for the choosing of the next pilot, they get a new pilot right then and there. And since we're seeing her past, yeah it's her.