I take your point about the ritual so it is highly unlikley Kabuto will find someone alive who he wants to kill and resummon, apart from Sasuke who I think is out of his league. But I don't agree about the contract thing with Edo Tensei, as when Kabuto was demonstrating Edo Tensei to Tobi he killed that Konoha Anbu and brought him back using the ritual, with no signing involved. Unless you are saying there is an intermediary that executes the matching of the soul with the host body with whom a contract needs to be signed?
That said the Genjutsu is probably going to be the way to handle this particular situation.