He is the hogwarts headmaster. I figure they defied him in that capacity quite sufficiently. Heck, possibly even in his capacity as head honcho of the Ootp

The theory is that the prophecy was always about him - or at least that it was always a possible read on it, in the same way as Neville could have been the prophecied child. That is why it was spoken in his presence, not in Voldemorts. It isn't cheating, it is settling the open question. And well, he told people it was about Voldemort because letting it be known that you suspect you are a dark lord with prophecies about you is not very politically helpful.

Because it doesn't let him actually possess people? Being permanently intangible and voiceless makes him rather less of a threat. Or the darn thing is a monstrously powerhungry magic sink and his tests have used up dozens and dozens of horcruxes.. if it's a trap, there is a lot of things it could be doing.

And the mirror is easy, it can be programmed to let people go who believe themselves safe from it, regardless of if they are.

Heck, it occurs to me that they could be the true artefacts, just cursed - the point is that Voldemort was told about them quite deliberately, and so his use of them cannot possibly be to his benefit.