One other quick point and then to bed. What BB just described about the concave raytracing sounds a lot like a job for Shadow Min Bias on the light source to me so I gave it a try. I know the manual makes it sound as though Shadow Blur Radius and Shadow Min Bias only work for DM shadows, but it's not true as I discovered some time ago. So, both images here have Smooth Polygons turned back on for the backdrop, and, of course, for the whole render. The one in this post with artifacts has Shadow Min Bias set as delivered from **pjz99** at 0.1 while the other post has it jacked up to 1.0. Looks clean to me.

Whichway