Yuri has basically never been fooled. She calls out Otonashi to explain his theory to the rest of Battlefront, then lets everyone make their own decision.
Meanwhile, she investigates. Whoever is controlling the shadow monsters has reprogrammed the NPCs.
It stands to reason that they used a similar method to how Tachibana built her powers. Yuri investigates the computer lab.
It turns out someone has been stealing computers. And there's a suspicious underground passageway leading from the lab to the ruins of the Guild.