I have, unfortunately, seen the episode more than once, and the reason I think John Hurt maybe the Valeyard is because the Whipser thingy got dumped into the Doctor's Time line, and attempted to as the, Whispers did, unravel what the Doctor did and steal his life. I am hoping they do not unravel Eight's time, but that is the other alternative. He could also become the Time Lord Victorious which might be interesting.

Paradoxes make me squirm because they are thrown about in sci-fi like jelly babies. Any time travel is filled with tiny paradoxes. People from the future or past moving a pieces of dust, or taking up space--it is an immediate risk. The Major science problem is how long someone can actually travel to a time period and not alter it.