Short answers, pulled from the story.
Moral nihilism is the metaethical view that nothing is morally right or morally wrong and that morality does not exist. Unlike moral relativism, which allows actions to be wrong relative to a culture or individual, nihilism holds that there is no wrongness anywhere. Unlike expressivism, nihilism maintains that moral claims do try to state facts; they simply fail because the facts do not exist.
J. L. Mackie developed error theory in his 1977 book Ethics: Inventing Right and Wrong, though the position was prefigured by Axel Hägerström in 1911. Error theory holds that moral judgments sincerely try to describe moral features of the world but always fail because no such features exist.
The argument from queerness, formulated by J. L. Mackie, holds that objective moral values would be entities of a very strange sort, utterly different from anything else in the universe. Their strangeness gives reason to doubt their existence. Mark Timmons reconstructed this argument in his 1999 book Morality without Foundations in terms of philosophical naturalism, noting that objective moral properties would be both metaphysically mysterious and would require a special faculty of perception that science has not found.
Mackie holds that moral claims are false because no moral properties exist to make them true. Joyce argues instead that moral claims are neither true nor false because they suffer from presupposition failure: they assume the existence of moral facts that do not exist, the same way a claim about the present king of France presupposes a king who does not exist. Joyce calls his version of this position fictionalism.
Richard Garner argued that if there are no objective morals, engaging in moralism is a deceptive behavior that harms epistemological integrity. He advocated an alternative he called informed, compassionate amoralism: a blend of compassion, non-duplicity, and clarity of language intended to nurture tolerance, creation, and cooperation. He believed this approach would also make human conflicts easier to address and resolve.
Not necessarily, according to several of its leading defenders. Mackie and Richard Joyce both defend continued use of moral talk and action even in knowledge of its fundamental falsity, viewing moralizing as an inherently useful practice. The legitimacy of using moral language while holding nihilism is itself an active subject of debate in philosophy.