Politics is difficult. It is not a game that requires hastiness or the addition of a lazy man's thoughts. If one is not careful in the dealings, failure is what awaits. And failure in politics could spell literal doom not only for the character but also for the family members.
Duke Grosskom had committed a blunder. Any later and it could end up biting him hard in a very bad position.
He limited his sight only to the picture he wanted to see and forgot to look even wider than what he was shown.
Prince Talen is favoured by all the nobles and also has ties to Bastoral kingdom. Naturally, when he succeeds the throne, he will get the full support of Bastoral kingdom. That would put him on a track backed with powerful support.
While the Grand Duke has over half the authority of Emedia kingdom, he does not necessarily have all the nobles at his beck and call.