After a large war with high civilian/peasant casualties, I’d assume that agriculture would be disrupted. In order to get more people back in the fields, would it be feasible for people living in city slums to be offered farming land if they were willing to relocate to it? They’d lack the knowledge required for farming, but, realistically, could that be taught to them in large numbers? If so, would the lords of cities filled with the poor consent to it, glad to get rid of them?

Offering vacant land to new settlers is a historical thing that happened, but not particularly city-dwellers. Remember, pre-modern societies were (for the most part) almost entirely rural. 

So instead what you’d see is offers being made to landless agricultural laborers, younger sons who aren’t going to inherit the family farm, farmers from neighboring regions.