For all their power and abilities, they don't seem to realize they've been infiltrated by a guy who's been enslaving and using spirits until they die.

This entire story took a blatant nosedive the moment they introduced another "cursed child" which is the villain this time. Especially since this breaks the entire universal setting the title itself is based upon, which is the fact that in this world kids like MC do not live past childhood and die because of excessive mana, and MC was only able to survive because he was picked up by the spirit queen.

Up until MC, there was zero instance of such a thing happening so the entire society calls people like MC who have black hair and eyes as cursed children. This is why it makes entirely no sense to make the villain another cursed child. It breaks the "MC is unique" premise completely.