A few quick answers:
1. If you play the cotton RGD, you only get $15. If you don't you may get $50.
2. It's once a turn.
3. Don't remember exactly at the moment, think it's like 25% each turn.
4. Every time there is a "rot event", which is like 10% I think, 1 unit of cotton rots, and is removed.
Every time a unit of cotton is sold or rots, it disappears, so the number on the RGD is the number of cotton units you have left.
Remember - The beatings will continue until morale improves.