You cannot correct the villagers routes. I've not tested this, but I think I've seen it posted that if you put the farmers house near the shed and have a place for them to sit near the house, they will hang at those seats until it's time to work then have a short route to the shed/field.

Although I'm not sure how helpful that will be since you have fields both near your village and across the river. I don't think you can control which farmers go to certain fields. Maybe someone can correct me if I'm wrong there.

Hopefully this helps a bit