So, I think this is it:
The blonde (I forgot her name) is possessed by the blonde shaman (the one who talked to the Crown Prince). Do you remember them saying that they entered the Nan residence? Maybe they used this ability and controlled the blonde girl's mind, like, it's safe for you to use a girl who came out of nowhere and then thacharam (pikachu face) she is actually an enemy. So far, my theory is that this blond shaman used mind control / body and mind possession and then used the blonde girl as a scapegoat for the Crown Prince's plan.
With her story being an isekaied character, I think it's probably more like shaman using a girl completely out of nowhere because of the betrayal (like a friend who cheated on Nan-er), you could even say that she just approached the Nan-er to kidnap her. It's the perfect pawn for your plan, and you know, corpse doesn't speak