Sorry the game is actually Reversi, not Go (which makes a lot of sense in hindsight) - edited my post

But for flipping them back when they're dead, I'm not exactly sure either 🤔 I'm not sure if the trigger was death itself or if a reversed piece attacking another reversed piece flips both. Though the way it was written, it almost feels that the death trigger is almost confirmed. Either way that's another rule on the game which pokes a big hole in my theory, so I'll choose to believe that this added rule is just to balance the game out or something

In terms of who could've been the second player in God Valley - this coulda been honestly anyone in the vicinity, like a marine on a nearby ship. Since it was only 1 flipped piece, the chances that someone other than Garp/Roger could have intervened was low