OK, so with a light tug on the pin-wheel, it comes off the shaft. At this point I'm absolutely sure the crack is not supposed to be there. No obstructions around the impeller shaft, pin-wheel, shroud, and magnet. Definitely no grains of FeO2.
It is apparent that the shaft is oxidizing, causing the shaft to swell, which caused the crack in the pin-wheel, which is allowing the impeller to spin off balance. Hence, the spots of wearing on the impeller.
Either Titanium rusts, this shaft is not Titanium, or there is something in the material in the pin-wheel which is oxidizing.
Here are some more pics: