if Katsuragi's physical body is not dead, and his part of the conscience has switched into Sento..technically we could have two Katsuragi lol, one being stalk (evil Katsuragi) and one being Sento (good Katsuragi)
But I think that "Sento is Katsuragi" part pretty much spots on (I really hope your theory is right). That's why the assistant said Katsuragi is "alive" in the last episode