Hmm, cool idea about the nonverbal spell and stuff, JK never did say what would happen if they did it at the same time.

But what spell could he have used to have said the killing curse out loud but still keep Dumbledore alive? *shakes head* Nope, I agree that the argument between Dumbledore and Snape was about Snape killing him when the time came, but I also believe that Dumbledore is dead dead.