If I breed a Female Durant with 31 Attack IV's holding a Power Bracer with a Male Durant with 31 IV's holding an Everstone, what are the chances that the offspring will have both 31 Attack and Speed IV's?

If I'm figuring this correctly - about 9.5% (9 out of 85, specifically).

The Attack IVs are guaranteed, because of the Power Bracer. The Everstone guarantees the nature from the male Durant, but that leaves the IVs up for grabs. The offspring will get two of the remaining five EVs from one, the other or both of the parents. Out of a pool of five stats and two sources, that's 10 total stats, from which 2 will be chosen. That's 90 possible permutations (according to

this), of which 5 duplicate the same stat from each parent (Speed from both the father and mother, for instance), which is impossible. So eliminate those 5 and there are 85 possible permutations, of which there are 9 each with a given stat in a particular position - Speed from the father, for instance. So - a 9 out of 85 chance that it'll get the Speed from the father.