1) dB can be positive or negative. It's often negative for noise levels and positive for gain or signal-to-noise ratio.
2) dBm is the signal power relative to 1 mW. Thus -10dBm is 10dB below 1mW or 0.1mW.
The noise is given per Hz (a 1 Hz bandwidth) so you can calculate the total noise in a given bandwidth because the total noise is proportional to the square-root of the bandwidth.
3) Resistivity is given per distance for a (theoretical) infinite area of the test material. This means the measured resistance is simply proportional to the distance between the test probes (in this case in cm) times the resistivity.
4) A epi wafer has an epitaxial layer, whereas obviously a non-epi wafer doesn't.