Enquanto eu leio (há apenas algumas semanas), você geralmente tem razão.
The sand came from the constant blasting of the mining operations, and yes, Victoria did set off the pit of the nuclear bomb the "bandits" had dug up, destroying at least one city of the mining society. Whether that ended the mining is doubtful, in my mind, and what happened to the others seems to have been intentionally left as a loose end (let's not forget that his Wool Omnibus turned out to be the first of a trilogy) -- as the narrator of a movie once voiced, "It's not whether they lived happily ever after. The point is that they lived."
Now for the bad news: freshly formed sand wouldn't act the way Howey wrote it -- what he had was more like the old, old sand of the Sahara, with grains rounded enough to act like a liquid in some ways, than the angular grains of fresh sand. And under no circumstances could a diver squeeze a nuclear pit rapidly enough to produce any sort of nuclear yield -- at most, they'd generate enough heat to blow the pit apart and strew radioactive fragments around the landscape, but to produce a classic nuclear explosion requires sub-millisecond implosion. It was necessary to the story, but it wasn't physically possible (any more than using vibration controlled by brain waves to dive a kilometer deep in fresh, jagged sand).