Hi Colin!
So sorry to hear of your misfortune, and I hope things will stay sorted from now on.
Please, can you tell us what tire you were using in the rear, and at what inflation pressure?
Can you also tell me if the rim was drilled for presta, or if it was redrilled larger in diameter for a Schraeder valve?
Pressure and valve drilling don't sound like culprits in the rim-joint misalignment, but when I heard about the crack near the valve, it made me wonder if there might be other contributing factors to the failure itself, if the rim happened to be already somewhat marginal.
All the best,
Dan. (...who always likes to autopsy failed components)