Thanks guys. Yes, I'd love a way to get rid of overlaps, but I'm afraid that's not possible without some major surgery to the software. You just need to build up the lateral volumes carefully.
The light is very simple; I used to project an image map through an extra camera located at the location for the lit area, directed at the sun, but the projection is always with a certain angle, and it leaves a narrowing beam at the camera location. Which is kind of opposite to what you'd expect.
Now I use a rotated simple shape (soft circular), which gives out a straight beam. I know, there are no handles, but you can use a transform shader, and figure out the angle from sun parameters. I'll make it easy for you; for a sun at 300 and elevation 25; set transform shader Y to (270=west) +30, and Z to (90= straight up) -65.
Both used as masks for cloud fractal or final density.