Error theory is an extremely implausible hypothesis. Are you absolutely committed to the thesis that the wanton torture of innocents is amoral? — darthbarracuda

Why is it implausible?
Amoral is a slippery word to me. I would rather say it is false that torturing of innocents (whatever that is) is wrong (or right).
I don't think the difference between a moral error theorist and a non-cognitivist is that big. Moral error theorists claims moral utterances is about objective moral facts which simply doesn't correspond to anything in reality. Non-cognitivists say we are not talking about objective moral facts when we utter moral sentences, we are simply showing our approval or rejection for something. But since, for the non-cognitivist, this is what moral languange is they can still claim to be firm believers in morality.

I agree it is kind of a problem for error theorists coming up with a theory on how you should behave. The most plausible ones seem to ground themselves on some variant of what is good for the individual (some kind of enlighted egoism). But this of course assumes a person remains the same over time which is not obvious to me.

Although i would suggest that people often blurr Moral Nihilism with Nihilism. It is possible to be a moral nihilist withput bein nihilist. — Gotterdammerung

What is the difference between nihilism and moral nihilism?

For the OPs original question I would need to have some information about size and magnitude. Was world war II a nuclear war since two atomic bombs were used? Would it be a nuclear war if two countries sent like 3 nuclear weapons at eachother? Or is it just nuclear war if all countries would nuclear capabilites send like all their nuclear arsenals?

So to answer the question preliminary I should say I'm going by the more deaths/the closer to human extinction the worse.
I do think the question is harder to answer than it appears. While the deaths of the direct explosions of course would favor an explosion in Yellowstone over nuclear war, the more longterm issues makes it harder.
The big problem with both an Yellowstone explosion and nuclear war is the risk that the atmosphere would fill with dust/materials from the explosions/burnings which would wreac havoc with the global climate and cause vulcanic/nuclear winter. Depending on size of said explosions (and in the case of nuclear war how much infrastructure would be set on fire and cover the atmosphere with dust) we could be looking at a decade of sharp drop in temperature leading to drastically reduced harvests and causing mass starvation world wide and collapsing infrastructures and societies.
And here we may have an argument for preferring nuclear war. If we say both scenarios lead to mass global starvation on an equal scale I do believe it would it would be better if nuclear weapons were gone since the possibility they would be used in a situation of global scarcity seem to quite high. If the the were used it would of course exacerbate the problem and the risk of human extinction would be much higher.

(Whether or not there is (intelligent) life on other planets (and then how many) is very interesting, but I think deserves a thread by itself, so I won't get into that discussion here.)

I find your post a bit confusing. If you are a ethical nihilist (I'm taking this as you saying there is no right and wrong) why do you think it is problem to justify your political position? Of course you can't justify it from an objective, rational point of view since you don't believe there exist one. On the other hand there is no way to dismiss it either.
In discussions you could say stuff like "from an utilitarians perspective..." or "according to Kant's...". So you take different positions and lie out the consequences of them. But I guess that won't be enough since you seem to be searching for an actual guide on how to live and make "genuine" moral and political statements.

I think it is a mistake to think that just because we can explain the historical development of (some of) our feelings we can reduce them to the evolutionary function they have had. Just because in evolutionary history it might have been in the evolutionary interest of the individual to be altruistic doesn't mean someone can't be a genuine altruist. To see if an action is altruistic we need to look at the motivation of the person, and not how this motivation has historically evolved. (I guess a comparison with the sex drive can be made. Evolution has developed the sex drive to make us create offspring. But when we get horny we often doesn't want to create babies, just have sex.)
Maybe I'm reading more into your post than what you are claiming here, but I felt I needed to get that out of the way.

So to my most constructive part of the post. Spinoza might be a philosopher to your liking and especially his Ethics. Nietzsche did like Spinoza.* Spinoza doesn't reject good and bad (normative judgements), but he does define good as "that which we certainly know to be useful to us" (Ethics, IV Definition 1). In short what is good for us is expanding our own power by gaining knowledge of the world which will make it possible for us to be as much as the cause of our actions as possible (as opposed to just reacting to outside forces), but this is enhanced by others also gaining power/knowledge which leads to a situation were it is beneficial/ethical for a person to help enhance the power of other people. So Spinoza is creating an ethics built upon the enlightened self-interest of the individual.

Ps: Was quite tired when I wrote this. Hopefully it is quite coherent.

* Quote from Nietzsche: “I am utterly amazed, utterly enchanted! I have a precursor, and what a precursor! I hardly knew Spinoza: that I should have turned to him just now, was inspired by “instinct.” Not only is his overtendency like mine—namely to make all knowledge the most powerful affect—but in five main points of his doctrine I recognize myself; this most unusual and loneliest thinker is closest to me precisely in these matters: he denies the freedom of the will, teleology, the moral world-order, the unegoistic, and evil. Even though the divergencies are admittedly tremendous, they are due more to the difference in time, culture, and science. In summa: my lonesomeness, which, as on very high mountains, often made it hard for me to breathe and make my blood rush out, is now at least a twosomeness. Strange."