I think if the stones were apt to return to their original form and location, just upon being temporally restored, they never could have been taken out of that timeline in the first place. In fact, I think either the abduction of the stones would create alternate timelines for each event, or the moment they took the stones, everything in the original timeline would have changed to the point that they wouldn't have made the trip back in time to begin with.....They should watch more Star Trek, before messing with time.