Lately we've had a rash of hacking/exploiting involving players using the argent cannon in Wintergrasp to effect an unnatural outcome. My suggestion is to disable the argent cannon even if it makes a quest undoable so we can bring fairness back to Wintergrasp. Players are already having to deal with multiboxers there, but this argent cannon problem makes Wintergrasp essentially unplayable. The cannon can even spawn kill players at their graveyard, making it impossible to play.
I think we can all agree it would be for the best to fix the vehicle in question rather than to break a quest. Also, if you haven't done so already, please report it on the bug-tracker.