Which jobs have occupational hazards

Well like you said, Farmer is 1 of them. They will only do their farming from early spring to early autumn. Besides that, traders only do their job when a merchant is present. I personally keep the farmers on their job at all times. And take the traders off their job once there is no merchant in sight.

Finding the right balance for your farmers/traders in the "off-season" is really dependent on where you need them the most. If you need food, give them a gatherer job. If you need buildings send them off building.

I believe it's better to have many small stockpiles. But you shouldn't put it near labourers. It should be placed near where the recurring production is- trading posts, quarries, foresters/woodcutters. You want these people to store their resource output as quickly as possible so they will go back to working and producing. Some stuff like firewood, you do not want your villagers walking out into a blizzard because they're freezing and need firewood but the stockpile is far away. I apply the same principle to storage barns.

The real problem is for resources that you can't reliably produce locally like iron or stone. For these resources they will have to walk a long way to get it from whatever your current source is, and there's no stock pile placement to alleviate this. But you can place your stockpiles to increase production. It's a lot easier to build more blacksmiths than to summon more merchants to get more iron.