It would be interesting from an evolutionary standpoint to see what "defenses" the halflings and other creatures will develop to counter the extreme toxicity of the world as it stands. Halflings have "free(ish) will, of course, but it would be strange for them to be living unchanged in an environment where they didn't evolve defenses to the "natural" dangers around them.
Therefore, I humbly suggest a rare caste of mutant "superhalflings" who are dumb as rocks (and about as tough) but who produce offspring at a prodigious rate. Perhaps make these mutants have a lifespan of only a couple years as an adult and be incredibly naturally agile to survive the rigors of the world and keep the halfling race alive "in spurts" as it were?
Sorry to be wet blanket... While sort of cool, my take on all this is that despite environmental toxicity the world must remain playable for a vanilla human-like race (halflings) without enhancements like that. If we freely embrace an arms-race mentality, we'll eventually end up with a world of our version of hardened adamantine colossi trying to survive against interactions instantly nuking the entire embark site with fire burning at 150 million degrees urist (just let your thermonuclear fusion demon cause an instant material emission on every target on the map). Sort of awesome, but a genuine design problem because a) it constantly obsoletes and makes irrelevant the contributions of earlier players and b) reduces this to something that's fun to play for 5 minutes.
So that's why the rules say it must remain playable with any vanilla-like race choice and I think not hardening the halflings makes them a good "base" race to test the world against. This means any attempt to make the atmosphere deadly will be rejected by the rules, even if there were another race that could deal with it.
Also I feel that if the world becomes too hostile it's time to take a step back and reconsider whether your contributions are actually contributions to the world at that point and not just "hey let's see if I can make players die faster". Furthermore if player reactions are like, this makes me not want to play, it's time to remove that part as this thing is pointless if it's just us demonstrating new raws we write and nobody wants to or can play the world anymore. But we're not there yet, all that's there is just more risks and those are good.
I'm going to mostly trust mod turn holder judgment on this one to simply not include completely broken things or ones that ruin player experience, even if they are posted. You probably want people to be able to enjoy your additions after all.