Does the attacker score before or after Ruined Rex's Deathknell resolves?

They score after your Ruined Rex Deathknell resolves. You cannot lose control of the battlefield while a chain exists with a Deathknell on it, so they cannot conquer and score until that chain is empty. After combat damage and any resulting chain items resolve, the game checks whether only one player still has units at the battlefield and then establishes control.