I agree -- this is central to figuring out the next few chapters. My best guess is that Harry said "Hello, Voldemort" so that an invisible Cedric Diggory could overhear. Diggory could time-turn one hour back to tell Dumbledore at 5:55 or so, or, alternatively, could inform Dumbledore after 6:55. Dumbledore himself has a time-turner, so as long as Diggory gave no information from after 7:00 Dumbldore could travel back 6 hours from then. Dumbledore knows that Harry is Riddle, so he probably has some aces up his sleeve.