Dynamite,
I will try to answer a couple of your questions.
EPR as you correctly state is a measure of the thrust produced by a Jet engine. It is, as its name implies, the ratio of what comes out the back over what comes in the front. The inlet pressure P1 is sensed by a single probe (like a pitot probe), which is usually inside the engine inlet cowl, but can be on the spinner (B727) or the pylon (B707). The exhaust pressure P7 is sensed by a manifold or rack of tubes with holes in the jet pipe.
In the main all EPR indicating systems will, assuming TO power is acheived before the a/c is rolling, show a decrease in EPR as the a/c accelerates down the runway. and this is quite simply because the pressure at the inlet probe increases as the ram air effect is sensed at the probe. A/c with the probe mounted in the inlet will only show a small decrease because the probe also senses the pressure of the air being sucked into the engine. The old 707 however, with its probe mounted on the pylon, away from the air being sucked into the engine used to show a marked reduction in EPR as it trundled down the runway.
Cost index is a figure provided for the a/c usually by the airline planning and performance dept. I can't give you any specifics, but it is used by the FMC to compute optimum climb and cruise performance levels. A low cost index will cause the FMC to calculate performance levels to provide a lower fuel burn. With a higher cost index, fuel burn becomes less important and speedy enroute times more important. The final figure is a compromise between fuel burn and enroute time and may change depending on the route being flown.
INS, ahh, that old chestnut. The first and most important thing to realise is that neither INS or IRS use gyros to sense north. The gyros are merely there to either electronically or mechanically maintain the INS/IRS platform level and aligned with true north.
The system senses north in the following way:- there are 3 sensitive acceleromerers each aligned 90 degrees relative to each other. We'll call them the vertical, lateral and horizontal accelerometers. During the initial alignment process, the outputs are used to calculate the vertical axis (in the old INS systems the platform was actually moved by motors so that the vertical accelerometer was physically aligned with the vertical). Once this is known then the outputs from the accelerometers are used to sense acceleration forces due to the rotation of the earth and it is the resolution of these forces that aligns the INS along the true north/south axis. Note that due to the obvious equitorial ambiguity (acceleration forces at 40 degs N and 40 degs S are the same) it is not until the a/c present position is put into the INS that true north is known.
Hope this helps