[m5james]

1) If the injector stuck open, it's dumping fluid, so just like water, it hydrolocked and something had give.
2) I'd say yes, but they clearly started cutting corners wherever possible after having to foot the bill for an engine. On the I6 engines fit example, if one injector failed in a bank (1-3 or 3-6), they'd replace the entire bank. Did they fail on both sides of the engine? Did only one bank of injectors get replaced?
3) It seems ignorant on their part to replace only the defective ones seeing as all the others are old and likely to fail as well. I hope their warranty has a warranty!
4) I'd gotten really low mileage and black pepper sooting all over the back before mine failed. That sooting was unburned fuel, which also washed the turbo and catalytic converter completely spotless. Raw fuel clogs cats, so I gutted them while I had everything apart.