Looks pretty much where I'd got to - apart from the fact I hadn't figured that you didn't need to edit ground_types - tried making it totally black last night and everything still worked.

Noticed one mistake you might want to edit in your post - should be disasters.tga, not map_disasters.tga - I currently have this one totally black too (R:0 G:0 B:0)

map_trade_routes can be totally missing and it will still work.

Also, you miight want to make it clear that disasters.tga, map_radar1.tga, map_radar2.tga and water_surface aren't used to make the map.rwm.