Philosophy isn't the only motivation. If you don't have philosophical motivations to do something, it is likely that expediency or self interest will fill the gap. Expediency would tell you not to pull the lever in case you were charged with murder.

This is SO different from my experiences and observations about people's decisionmaking that I'm not sure how to adjust my models.

When someone encounters a (moral) decision in life, their choice often depends on their personal philosophy.

Disagree. Their choice is based on a very complex learning system, and their personal philosophy evolves to explain their choices. There probably _IS_ a feedback loop where repeated use of the justification encodes the decision weights more deeply, so the philosophy does appear to guide future choices. But causality definitely ain't that simple.

But if your dominant belief is nihilism it will tell you that it doesn't matter what you choose because there is no right or wrong answer

No. It will tell you that it doesn't _objectively_ matter. There's no right or wrong _outside of yourself_. That doesn't mean there's not preferable and dis-preferable choices, nor that you can't have personal judgements about the likely results. Nihilism doesn't deny consequentialism (nor support it; you can make choices based on your evaluation of yourself, rather than your evaluation of the consequences).

The uncertainty argument doesn't work either - Pascal's Wager fails because one can imagine a pull in every direction. There are an infinity of gods and moral theories, each of which having infinitesimal probability. Without evidence on which one(s) are more likely than others, they cancel out. If you don't believe in any specific other moral stance, you do _not_ have to act like you do just because you might be wrong.

For those of you who are wondering why I decided to quickly explain the trolley-problem: the reason is because I want you to be able to send this to non-LW friends. I myself struggled with nihilism for a long time before I thought of this argument. A less jargon filled post might make it easier to help other people put nihilism aside as well.

This argument has long been known, and much discussed. See e.g. Jacob Ross, Rejecting ethical deflationism and William MacAskill, The infectiousness of nihilism.

What you are describing is a passive sort of nihilism. Active nihilism, on the other hand, would actively try to negate the other values. Imagine a parliament where whenever a non-nihilist votes in favor of X, a nihilist votes against X, such that these votes exactly cancel out. Now, if (active) nihilists are a majority, they will ensure that the parliament as a whole has no aggregate preferences.

I think that the attitude you're talking about is not what is commonly understood as "moral nihilism". You are saying "nihilism will say that there are no right or wrong answers to [the question what to do]", but even the question "what should I do from a moral point of view?" is irrelevant in the moral nihilist stance.

I'd say that you're conflating the non-existence of moral reasons (that's the nihilist claims) with the non-existence of all reasons. It does not seem like being a nihilist implies that you don't have any goal, as you point out below with the discussion on existentialism. A moral nihilist (someone that holds the "meta-ethical view that nothing is morally right or wrong", from Wikipedia) could still have goals and desires upon which to act.

The various voices would not be "nothing is right or wrong" vs "pull the switch", but rather "do according to your own values" vs "do so as to maximize well-being" or even "do so as to do what is moral".

The absence of values can not matter indeed, but moral nihilism is not the absence of values rather than the denial of the metaphysical nature of these values.