You should allow some room for the air to circulate under the tank. We use little rubber pads called "bumpers" that are supposed to be for stereos, VCRs and computer equipment. You just stick them to the bottom of the tank.

Yes, the thermometer should be closer to the bottom of the tank. In fact, I my probes sitting right on the substrate. If you're using an analog thermometer as opposed to digital, your readings could be inaccurate. The $15 indoor/outdoor Accurite thermometer/hygrometer has been recommended here many times, and that's what we use at our house. I was shocked at the difference in the readings.

If you can maintain proper temps (80 cool side/90 warm side) without the overhead light, pitch it.

Hope that helps!