OK, so here's what I learned.
I started off by creating a light fixture that had an internal light source, and a metal frame. I attached lighting attributes to the light source, and the first picture was the result. As you can see, the metal frame cast horrible shadows over everything. I also set the material of the inner light source to the white-light emitter material. I don't know why it rendered as transparent; it looked fine in the preview mode.
In the second picture, I made the entire light fixture the Renditiioner light source, while leaving the materials alone. Much better.
In the third picture, I set the lighting to "Interior > Daylight sunset". It came out pretty nice, and you can see that the two light sources have cast reasonable shadows, but the light fixtures themselves have been rendered as grey rectangles. I don't know why.
In the fourth picture, I set the lighting to "Interior > Daylight night". As you can see, it's horribly washed out. I still don't have a grasp on what I'm doing wrong.
I edited the light attributes and set the interior light intensity from 1.00 to 0.10. Now it looks ok. See my next post for more.