"So.... If we reshape the orbit of an ice comet... providing the "O", and torch off a 1 megaton nuke, how long would Titan burn? "
Until there was no more methane or oxygen, of course :p
Although, wouldn't the impact by an ice comet provide the friction and heat required to ignite it?
Even an Ice comet has it's oxygen locked up in water.
Even with an impact I don't think you're going to separate that molecule.