I'm betting on the Black Knight being her fiance in past life. We just need to see if he has scar under his helmet. If he is who I think he is, then perhaps the assassination plot wasn't really started by her adoptive father.
Considering knockers are exterminating everyone to eliminate pain, it's possible that left arm's knocker started killing everyone because it couldn't bear seeing them suffering.
He first transformed into the girl, so the damage that killed her was healed, then separated from the body. Previously he would just create the body, including fatal damage. Maybe the soul can only move into body if the body can live, so that's why previous attempts didn't work.
@SotiCoto She was the first real leader of prisoner colony, she's definitely an important historical figure now if that nation survived for all these years.
I feel like he will not be able to win against her, so he will do the ultimate chad move: drop his stubborn pride and confess because it's more important to spend time with her than winning a board game.