Let's address your last question first. You wanted to change the unit distance and intensity so that there was no net change, right? So you doubled the distance in order to enter half the intensity? I get it, but its wrong. When you double the distance, the corresponding intensity is 1/4 what it was, not 1/2. Remember - it's R SQUARED? You should have entered 12.5%.

As for why the ceiling is dark, it sure looks to me like the ceiling color is a very very dark gray. Did you look in the ceiling shader? What color is the celing? If you place an ordinary spot light shining up on the ceiling what does it look like?

Perhaps the dome is lit because you have not enabled shadows? Did you enable raytrace shadows on the point lights? Did you enable ray tracing?