1) I think you're right, it was someone with a 318i, but the body style is still the same. If you have fold-down rear seats, you might be able to finagle the latch without resorting to surgery.
2) You do need access from inside the trunk to remove the rear light housing.
3) Quite possible it was the gas, possibly with water contamination. You might want to move up the time for replacing the fuel filter, or put in a bottle of dry gas and a couple of tanks later a bottle of injector cleaner.