It seems to me that there is a lot of confusion on this thread about the difference between Deviation and Variation. If you understand the difference then the question in post 3 is pretty clear.
OK, back to basics then;
Maps and charts are produced with reference to true north/south, because this doesn't changes.
Variation is global and relates to the earths magnetic field (flux) however owing to the earths rotating wobble it is continuously moving side to side East to West/West to East. A magnetic compass aligns itself with the earths magnetic flux
not the True North/South poles around which the earth is said to rotate. The angle of difference can be measured but it varies (variation) depending on where you are on the earth. On the chart, be it the 1:500,000 or the 1:250,000 version, they overprint it with a dashed 'Isogonal' line. At any point along the line the variation is the same. This makes it straightforward for the pilot to plot. As the flight progresses it is necessary to update the magnetic variation value from the chart Isogonals and the magnetic compass heading should be revised for each sector.
Deviation, that concerns a pilot, is a local problem; in the aeroplane electrical circuits such as wiring looms and equipment in particular are the culprits. Don't put anything that is magnetic near the compass. A 'compass swing' (see the compass card example above) is done with the engine running and all electrical equipment switched on. Before starting the engine and switching the master switch on note the compass indication. After starting and switching on the electrics see the difference in the compass. Incidentally, a walker needs to be aware that the surface itself may magnetically be deviating, in localised patches, and will change the compass needle. Deviation then can be described as a corruption of the needle swing but this effect can be known, measured and recorded.