You replaced the head gasket and inspected the head so I'm assuming the motor had previously overheated? Were you able to achieve proper torque when tightening down the head? I'm wondering if there wasn't damage to the block and if the head studs have now stripped out of the block allowing the head to lift (common problem when these engines overheat and why BMW recommends replacing the block). Running the car and then shutting it off obviously heated up the coolant and caused it to expand and pour out into places it shouldn't have been, one way or another. I wouldn't be surprised if you even hydrolocked the motor.
It's extremely rare for a timing chain to break though. Doesn't sound like you followed the proper procedures for timing the cams but worst case scenario there is you could have bent some valves. It's not obvious to me that there is any way this could have led to a broken timing chain and especially not the secondary (upper) chain. Only thing I could speculate is that the massive cooling leak flushed the cam bearings and caused one or both of the cams to seize in place. Perhaps it was just a random failure or perhaps it was related to the overheating but if everything is properly rebuilt (or the motor ends up being replaced), I wouldn't worry too much about future timing chain failures.
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