2. Supply wagons are used to construct depots. When they are used up, do they reappear back in the force pool?
Usually yes, unless you started the game with more wagons than your build limit. If you have supply wagons available for recruitment, then they should always reappear in the pool. But if you side starts with, say, 8 wagons, and your build limit was set as 6, the first two wouldn't reappear. Same applies to any captured supply wagons.