Hello Bladex,
Charcoal was known in the Antik (source Wikipedia), for coal I'm not sure because it has to be mined. But there must be some places were it was easy to see and find or how would it have been found at times without our modern devices?
The important point is that if I want to make a difference between coal and charcoal in Civ5 I need to create a specific Charcoal resource.
Building can eventually be coded to use either coal or charcoal but not units (it was possible in Civ 4, one more thing to miss !
Now if somebody can find out since when Mankind is using Coal this would be interessant to know! (i'm to lazy and tired to search right now !)
Snow is only esthetic in the game. Mountains are required to build the ski resort but there is no possibility to bind this building to mountains with snow.
But this could give a great Civ 6 : weather and season changes with consequences for units moves, that would be great !
The aircraft factory is required to build aircraft, only the WWI aircraft do not require it.