The letter invitation fits best, and the meeting need not be explicitly with the king or first prince to take some pressure off. The kidnapping should be recent in everyones mind, within a few days at most a week. The letter need not be terribly personalized, think of it like a formal request from the king, delivered to hand picked knights. The meeting host could be the knight commander general, (A military advisor to the king) this would make the meeting more meet and potatoes, then towards its end, the Prince, or king, or both could show up to say his part, instilling the whole thing with a sense of dramatic effect. But I look forward to what you come up with, this is just my personal take.