The setup is kind of weak. The problem is, how close can this guy really be to the princess if they hardly knew each other? It seems like the engagement was a "reward", in that it would elevate him to a royal, but that undermines the whole betrayal. He was mostly using her. She didn't care for him, so she found a way out of the deal. Him feeling betrayed rings hollow since he was forcing himself on her in the first place.
It would make sense if the king offered the princess up as a prize for whoever could complete this suicide mission, never expecting to have to actually pay up. He just wanted to keep the barbarians occupied, and didn't think the campaign would succeed. But then, why would he send his strongest knight order on this suicide mission? If he pushed for the marriage, it would mean he wanted the knight commander to marry his daughter. But if that was the plan, why send him on a suicide mission?
It then falls apart with the whole cover up, which again makes little sense. There were probably better ways to preserve the royal honor and the military strength of the kingdom.