Honestly I think he should have led with "I've uncovered a criminal conspiracy. Gildick is working with the enemy nation and poisoned mother." Since MC didn't know about the messenger arriving with news of the prophecy failing (I don't think he even knew she'd made prophecies about anything but the weather), he had every reason to believe he'd be treated like the previous times he claimed they chose the wrong Jewel Daughter. Whereas announcing Gildick murdered the queen would immediately draw in both the king and his brother, and doing something about him asap is critical.