Unfortunately, comets would affect the Earth's temperature in another way. In addition to dust and water, they contain a small amount of CO2, which would be released into the atmosphere as the comet melted. This CO2 would change Earth's radiation balance
Holding them to higher standards would mean prosecuting their failures more. Like a ton more, that part is obviously needed. But the threat of death penalty does nothing but satisfy lust for revenge.
While Skyrim probably has the same issues you ran into with Fallout/other TES games, it's quite viable to roleplay the game without doing the main quest without the game feeling empty.
I did one "playthrough" as an Orc blacksmith, with the goal being maxing out the smithing skill and crafting a dragonbone armour to present to a chieftan of one of the orc strongholds in order to join. I started out hunting deer and making leather stuff to gen enough money for food and board, working my way up, eventually venturing into dwemer ruins to gather metal. I did a handful of quests if they felt doable for a non-heroic regular dude and ran if I got into too much danger.
Strict pacifism wasn't my goal, I was just playing as a non-heroic normal person, but I'm sure you could do something similar. You just need to abandon the main quest and set your own goal.
Yer a skinjob Harry!