Roughly half of the population is misinformed about which alternative is objectively better. In that case, how do I justify a belief that I have a greater than 50% chance of being right, when everyone else has access to the same information?

"Voting is irrational unless you are arrogant?"

There are real differences in values, and by my vote I direct the outcome towards my preference instead of the other Virginia voter's. In that case, sure I want to vote, but should we really call it altruism?

You can still call it altruism, and it can be helpful to distinguish "selfishness" in the sense usually considered for decision problems from "altruism". The example I like to propose for illustration is the Codependent Prisoner's Dillema, which has Romeo and Juliet as the prisoners who are each obsessed with the other's wellbeing and the jailers use this fact when manipulating them. So when Romeo is "selfishly maximising his own preferences" and picking the option that puts him away for 10 years but lets Juliet go free he is also being "altruistic" towards Juliet while brutally ignoring her preference that she be the one who gets to be the martyr.