**Why yes, I AM a Rocket Scientist!**
Interesting questions.

As far as Headings and Courses between True north and Magnetic North, Toro's got you covered. It's a simple matter of adding or subtracting a specified Magnetic Variation (AKA Declination.)

The algebra for calculating Declination isn't too bad, but the formula usually contains constants that are extracted from mathematical models of the earth's magnetic field. This type of equation is known as a LeGendre Polynomial. At worst, the math is 3rd semester college Calculus (vector alegbra & series). I recall doing some of these calculations in Jr-year college Geophysics. For the most part, we were encouraged to use the wide variety of models and simulations available online.

Here's a website that will allow you to calculate the Magnetic Declination for any Lat/Lon/Altitude. It also gives you some other interesting Magnetic Field Parameters. The explanation that comes with the results page is High School/undergrad level.

It's a neat site. If you don't know your own Lat/Long you can input your ZIP code.

Compute Values of Earth's Magnetic Field (Version 4.0)
In terms of the Earth's Magnetic Poles, there is a Magnetic Lat/Long coordinate system. This is used only by the scientific community and shouldn't be confused with anything a pilot would navigate by. For shi+s and grins you may still want to try a model that converts regular Lat/Long (based upon true north) to MLat/MLong.

Geomagnetic Coordinate Transformation
You also asked about magnetic field disturbances and compasses. Again Toro is correct. Your compass is at the mercy of any signifigant magnetic field disturbance, be it a local disturbance in the Earth's Magnetic Field or Electromagnetic Interference (EMI) caused by electronic devices in the cockpit.

In most aircraft the magnetic compass is purposely located away from the radios and other electronic equipment in the cockpit. In fact, jets often have magnetic sensors for their compasses mounted in the outboard wing sections for this same purpose. (i.e. T-37 Flux Valve)

Most magnetic compass installations in aircraft have a card beneath them called the Compass Correction Card. Once a compass is installed in an aircraft (and periodicallly after that) a test known as a compass swing is performed. You turn on all the aircraft systems (avionics, engine in particular) and point the airplane at the known Magnetic North.

[Test is usally performed on a compass rose painted on the ground or with aid of a surveyor standing outside and away from the a/c. At Boeing, we have these large turntable fixtures reminiscent of the old train depot turntables. The jet is towed on to the turntable, which is then rotated for the compass swing.]

You compare what the compass actually says and jot that down on the compass correction card. For example if pointed at 360 deg Magnetic the wet compass says 354 deg, the pilot will know that with all the sources of airplane generated EMI, he/she must actually fly 354 to head Magnetic north (360).