

Hydrogen still produces water vapor which is only barely better for the environment than the carbon dioxide from gasoline emissions
This is completely incorrect. Even if we directed our entire industrial efforts to making water vapor, it would pale in comparison to a single hurricane. Anthropogenic water vapor is insignificant compared to evaporation from the oceans - which is incidentally why we need permaculture instead of industrially supplied water to fight desertification.
Water vapor in the atmosphere is part of how the greenhouse effect works, but not because humans put the water vapor there. As global temperature rises, the atmosphere can physically contain more water vapor per cubic meter without it turning into clouds or rain, and that causes there to be more water vapor.
Lethal according to this study? A sarong.
Properly lethal? Thick insulation, and undergarments stuffed with heat sinks. For example cold mud from a well, basement, or cenote, made relatively cold by nighttime temperatures and traditional air flow engineering. It probably wouldn’t last more than an hour, but that’s enough to travel. Then if that’s not enough, shelter in place. Spend daytime in basements and hope supplies last.
That said, the most practical and historically supported option is migration. Humans have migrated away from unsuitable climates for as long as we’ve existed. Disregarding politics, from an economic perspective “rural third worlder with no access to power or air conditioned spaces” is not losing a lot of production assets. Maybe if the first world stops being colonialist shits for one second, these migrants can benefit the common good more than before, when they’re given access to the means of production rather than being treated as detritus around the edges of colonial exploitation.
Imagine a billion extra farmers and engineers, ready to do the massive amount of labor necessary to turn habitable/temperate zones into sustainable climate-hardened permaculture. The issue isn’t technology or economic feasibility, it’s white supremacy.