If you are going to work in LightWorks, you need my book:
And you have not tried every setting.
1: window black
Page 21: Change LightWorks Rendering Method from "lousy" to "Best" - addresses the number of transparency levels calculated.
2: grainy
Supplement 10: Realistic Sun needs more samples to make smoothness- related to rendering size - bigger renderings need more samples. SLows it down.
Use "Sun Sun" Shader instead of "Realistic Sun" Shader for faster performance.
3: light cone solid
In the lamp object, turn off "Light cone in rendering"
Dwight Atkinson