Regarding: "rightism is what happens when you’re optimizing for surviving an unsafe environment, leftism is what happens when you’re optimized for thriving in a safe environment."

My suggestion would be that politcal beliefs in general are for optimizing survival and fairness. Both ends of the spectrum want the world to be safe. Both ends believe in fairness. But the threats are coming from different places.

Yvain makes a major assumption in his post that in apocalyptic scenarios people turn on each other. But this is something I would say we find more in films than in real life, except in cases of insider-outsider groups, like pogroms. At the same time, it is a defining factor in a person's political views. If we were to argue about this, we'd be arguing about politics, and I think we consider that off-limits here? Anyway, in the abstract:

It seems to me that the defining characteristic of left and right is how much authority, power, structure there needs to exist for there to also be order. "People don't rape, kill, and steal because the government/god/[structure] stops them." vs. "People don't rape, kill, and steal because they don't want to, for the most part." So the methods of optimizing for a safe environment with these opposing views point in opposite directions. Are the structures and hierarchies holding our society together, or are they its biggest threat? Build them up, or tear them down? Obviously, there are more moderate positions.

Basically, these are not safe vs. unsafe, but about the perception of where that source of danger will be found.

Regarding your last two points: "happiness is what happens when things are going well, depression is what happens when things aren't" and "the world is basically dangerous" etc.

Thinking makes it so. A person on the right perceives their enemies as far away, and weaker because their idea of an enemy is likely a foreigner in another country without a large military. They never have to personally interact with those they consider enemies (unless left leaning citizens qualify). A person on the left perceives their enemies as nearby, and stronger: the police, the courts, possibly many societal institutions and corporations, capitalism, sexism, their boss, etc. which are at least seen if not experienced in some aspect or another.

What I would suggest is, the closer and stronger your political opposition is perceived to be, the less likely you will be happy, and vice versa.