Am not aware of Water Pump pulleys coming loose as a particular problem with D's.
You may want to look at all possible causes of losing the belt. Pulley runout might be it, as your assistance fella suggested. Other causes to look into would be
- alignment of all of the pulleys: use a straightedge to see that each pair of the 3 pulleys under that belt are in the same plane, neglecting the wobble of the WP pulley for the moment. Sometimes an owner will install an alternator that isn't a direct fit re the pulley.
- belt age: was it new when the WP was renewed 10K miles ago?
- belt tension: you won't be able to tell directly now that it's broken but you could look at the old belt for signs of glazing on the contact surfaces. Low belt tension -> slippage/overheating/glazing -> premature breakage
Given that you breakage occurred in the Ozzie summer that last one might be interesting...
There's no reason not to try to straighten the WP pulley so long as your method doesn't kill the bearings.