Is Moral Nihilism a Morality?

[SenseMaker007]

After all, aren't the GOOD arguments also the most convincing? 

Unfortunately not. Almost everybody is more likely to be convinced by appeals to emotion or fallacies that argue in favor of what they already agree with than a genuinely logically valid and sound argument. Just read Thinking Fast and Slow. System 2 is precise and logical but it's the irrational System 1 that holds the most power over us. And as David Hume observed long before: reason is the slave of the passions.

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We've briefly touched on this earlier in thread, but it may be the case that no metaethical position properly understood and genuinely held lacks a normative function,

It is indeed the case that moral realism is true but that doesn't mean that all metaethical viewpoints are implicitly making normative evaluations.

not even in an error theory, as even pointing out error provides normative function.

But Error Theory is not nihilism. Error theory says that all moral claims are false.