What is the ruling on saying to an opponent, "You know best" or "Do as you please," with the intention of ending an argument? Will Allah hold one accountable for that? Is it permissible to adopt the opponent's point of view to expose his flaws?

Words are molds for meanings, and a command verb may be used for a threat, not for permission, as in His — Exalted be He — saying: "Do what you will; indeed, He is Seeing of what you do." And it is for threat and warning. It is permissible to adopt the opponent's viewpoint to show its flaws, as Abraham — peace be upon him — did to establish proof against his people.