You're forgetting one important thing.
Magic.
The Module states that for every 100 inhabitants of Ten Towns there is a Spellcaster capable of casting 3rd level spells (at
least 2 spells per day) such as NPC Priests, Mages and Druids.
Leaving aside any creative uses of spellcasting to help with hunting and survival (talking to/ locating animals, druidcraft cantrip, bonfire cantrip etc etc) there are some obvious solutions:
- Create Food and Water is a 3rd level Paladin and Cleric spell and it feeds 15 people. It's a 3rd level spell. Presuming roughly 1/3 of the Spellcasters in town are Priests, that's enough food for 50/500 people in the town right there.
- Goodberry is a 1st level Druid and Ranger spell, it produces 10 berries, with each berry capable of sustaining a humanoid for 24 hours. Presuming just 2 Druid NPC spellcasters for every 5 spell casters/ 500 people, with 14 combined slots per long rest, and that's enough food for 140/500 people.
Of the 1 spellcasters per 100 people, if 2/5 of them ae Druids and 1/3 Priests (and the remaining are Mages) and
you can feed nearly half of the population with spellcasting alone.
Also:
Create Water is a 1st level Ceric and Druid spell (however water shouldn't be a problem with all the snow around)
Purify food and Drink is a 1st level spell for Clerics and Druids, and it cleanses all food in a 5' radius sphere of all poisons and diseases. This would enable putrefying or poisonous foodstuffs to be cleansed and be able to be eaten (and it would be easy to store excess food in the freezing weather by simply putting it in a shed outside, where it freezes).
Druids capable of casting 3rd level spells can cast
Plant Growth, turning a field of even the most meagre plants into lush overgrown crops.
In addition it also does this:
So yeah. Magic.
It's likely that a substantial number of people in town are turning to Druids and the Churches for salvation in these dark times, and are getting crops blessed, and surviving on
Goodberries and food created by
Create Food and Water, supplementing any stored/ frozen meats like seal whale, axe beak, dog, walrus, reindeer and crag cat that they have left (which is likely running out).