Spark plugs can not explode.
If he had drained to coolant with the engine running, you'd have much bigger problems than you have now.
Nope. Nothing there that could generate enough pressure to cause this supposed explosion.
It's your car, you put the miles on it. If I'm wrong, why then did you request the oil change, coolant flush and tire pressure check? Odds are that your spark plugs are worn out and the wires need attention also.
You have it backwards. The stripped threads allowed the plug to be ejected. Since there is no reason for them to go anywhere near the spark plugs for the other services, it is your responsibility to pay for the additional needed services and parts.
And by the same token, the repair shop shouldn't have to pay to repair a condition that obviously existed before you brought the vehicle to them. IOWs, they'd be totally justified in pushing it out the door and leaving you to your own devices WRT what to do next.
Hogwash. You have no way of knowing that. By your logic, it wouldn't have happened if the sun hadn't come up that morning in the first place.
No, not in the least. depending on state law, they would then be completely justified in holding the vehicle until the debt for repairs was satisfied, and you wouldn't have a leg to stand on in court. You should be thankful that it chose to happen where it did, there is no better place for a breakdown to happen than while it's at the repair shop. Luck saved you having to pay for a tow truck.