Usami theory was mostly correct but he made one fatal mistake. He believed that the Seance user would be an average sorcerer and that made him oblivious to the impending threat. If he had known what Megumi had in his arsenal he would have pushed the anti-Megumi squad with greater force. For that threat hidden beyond hindsight was Hami.
During the Heinan period, there was no central authority that regulated jujutsu. In the absence of any law, sorcerers explored the infinite possibilities of sorcery. And some ventured into the darker aspects of it. Soon sinister techniques developed that could summon plague, turn humans into curses and to create undeads. All these taboo techniques were collectively called forbidden jujutsu. After Sukuna was sealed off, the three clans took it upon themselves to erase the blight on jujutsu. Manuscripts were destroyed and practitioners were executed. But some obscure bands did survive hiding their forbidden jujutsu for centuries. And the Seances were one of them. They had the forbidden technique to turn normal body parts of a human into cursed objects housing souls. But since no one could perform or teach such high class sorcery, the manuscripts remain obsolete.
For the Seances, fortune took a turn when Hami was born. She had a great talent for spells and had mastered all of Seance's technique at a very early age. And to the surprise of her grandma she could even read and understand the forbidden jujutsu scroll. She might be pathetic in combat but this once in a millennium sorcerer had the talent that could make the unreal come true.