I have experienced only two bearing falures: 1 on a Ford WIndstar that howled on every turn to foretell its imminent demise, and one on a trailer wheel that fused itself to the spindle like yours.

My guess is that the berings weren't 100% seated correctly?? Just a theory, since 2 years is an awfully short lifespan. I agree that grease mixing is an unlikely culprit.

If your mech continues to berate you, make him prove his virility by restoring the spindle using only a chisel, file and sandpaper like the old gas station mechanic from Churchville, South Carolina did on the trailer wheel in 1984. And he only charged $48, even after he busted his hand open on the job.