You do it linear to reduce the chance of seeing a swirl mark no matter what angle you view it from. If you use circular motions you will see marks in the car from any angle (assuming marks were put in). Using linear you may only see them from one angle. The general idea is to do the horizontal surfaces with a linear motion from front to back of the car. On vertical surfaces an up and down motion.
Pretty much yes. However, I find that using a good microfiber towel will almost negate the need to move in a linear horizontal motion, as it rarely makes a single swirl. Wiping in a linear motion is only treating the symptom, it is more productive to go after the source of the swirls--in this case the item that you are using to apply and remove the wax.
It doesn't matter how careful one is when detailing their car. Everything, and I do mean EVERYTHING induces marring to some extent. Don't believe me? Try purchasing a 1,000,000 candle utility flashlight and shine it on a dark colored car that looks "perfect" under normal lighting (even direct sunlight, asi my wife's black 330I).
As for MF towels not causing any swirls, the marring is too small to see immediately, but over time will become evident. It's not the MF that's causing most of the marring, but the 'invisible' dust that almost immediately settles on your car...even when detailing indoors. That's why everything should be applied, washed, buffed in straight front to back motions.....this will minimize the visibility of the 'build-up' of swirls that you induce every-time you touch your car.
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