I got the same problem: Cut = 0 Fill = 0 Cut not solid, fill not solid. It generates the cut / fill solids, but states they are not solid and 0 volumes.

Have tried scaling it up 1,000 times but still no joy.

In reality this is not practical as you state below:
"make sure that any proposed and existing surfaces are clearly different in their surface's heights/planes,"

Most people need to compare two contour terrain models, you can't change the contours just to make the volume plugin work as the design is set...

So for me I have paid $20 and can't get any useful results from it with using contour created terrain mesh solids

cheers