I repeat the way I handle this stuff in Other Worlds: if the NPC makes a successful diplomacy check against you in the royal court, saying we should invade Mordor, it does not mean you are convinced that this is a good idea, or that you must go along with it. It means that you were outmanoeuvered/boxed in. In the eyes of the audience at the Court, you lost. If you still decide not to assist with the invasion, or try to relitigate the argument, then the people around you will lose respect for you and your social capital will be harmed.