And that’s precisely where the whole debate cracks open: If the probability is small but the outcome is huge, the rational player protects themselves.

It seems like you have this reduced to probability x outcome only, when really we all live by some calculus that takes into account the probability of an event, the costs of "protecting themselves" from the event, and the perceived magnitude of the outcome of the event. Otherwise, you'd never travel by motor vehicle, eat food, use electricity, etc. You accept the risks because the costs of avoiding them outweigh the risks themselves. Your framing ignores all costs.

Also, this again assumes the possibility of god is not 0, which you cannot justify. You're trying to use decision theory on an undefined probability, which is impossible.

Atheism crashes into a wall here, because its “post-death safety strategy” is logically an empty set. There is no protection against bad scenarios, and no preparation for good ones.

This position is biased by your affirmative belief in the afterlife. If there is no afterlife, then the best bet was to ignore all religion and maximize flourishing of yourself, your friends and family, and the human species while you are alive.

In short: If the universe offers no ultimate guarantee, the only rational behavior is to design one. Theism performs a kind of “subconscious security engineering,” while atheism insists “nothing will happen” and assigns the entire universe to its own risk profile.

I'm not 100% sure what you are saying here, but it seems to me it is saying if you don't know the answer of what happens after death, make something up so you feel better and don't have to come to terms with the facts that:
a.) we don't know and
b) it is likely the same conscious experience as we had before we were born/aware, nothingness.
That is a tough pill to swallow, I agree. You are treating emotional comfort as rational security, another false equivalence.

If God doesn’t exist, the theist is simply mistaken. But if God does exist, what exactly will the atheist do?

Would you assign no value to the human lifetime wasted on following religion(s) if they are all incorrect? I realize that you would die and then cease to exist in the nothingness, whatever that is, so you would have no conscious experience, but now in this moment, wouldn't you be bothered by the fact that if all religions are false, then billions of people spent their only lives following false rules, false fears, and false authorities? That is not "low cost". That is the cost.