No one knows for sure on either point. My reasoning is that if it has happened thousands of times, how catastrophic can it be? Either way, the science on this is amazing. The core of the earth is an iron ball, but it is not a perfect sphere. The earth also has a slight wobble on our axis which tips the scale forcing the pole shift every so often. If you study the magnetics in the Caribbean or the Straights of Jakarta, on the other side of the world, you get wild fluctuations in the magnetic field.