First, understand a couple of things:

Even though we call these things "digital cameras," the process of turning photons into numbers is entirely analog. Analog circuits pick up all manner of noise from their surroundings.

Noise isn't one constant value, it's a range of them that top out at a level called the noise floor. The processing you did on the otherwise-black image stretches everything below the noise floor, most of which is close to black to begin with, across the entire dark-to-bright spectrum in each channel. The histograms before and after processing will show that.

What causes the white bit at the top of the image?

My guess is that it's thermal, perhaps the bottom part of the camera is a bit warmer for some reason than the top. Your processing will exaggerate this.

What is the partial circle apparent in the bulk of the frame?

That could be anything. The sensor could be picking up IR the low-pass filter missed and you're looking at your lens cap. It could be the result of deformities caused when the sensor was picked up from the back with a suction cup during assembly or minute amounts of stress placed on it by the retaining parts. I'm sure Nikon knows, but given how far down in the noise it is, it's either not worth caring about or it's been accounted for in the design of the rest of the camera.

What causes the vertical lines in the image?

Under these conditions, the best theory would be that some clock circuit in the body is inducing a bit of current onto one of the analog parts on a regular basis. If you check out this D80 IR conversion tutorial, you'll notice that there are components on the same board as the sensor and others close by on the main board behind it. There are lots of opportunities for that kind of leakage.

What causes the purple and green patches?

The sensors in digital cameras are noisiest in the blue channel, and if you look at the RGB composition of purples, there's lots more blue than red or green. The greener patches may be a result of some parts of the camera being noisier in different ways. Again, your processing exaggerates these differences. Most of the colors are actually so close to black that you'd never notice them otherwise.