It's usually at the ends that they go first, and it'll be hard to seal them where the nut is. Pretty sure you get a warning at the MOT now if there's something covering the brake lines that may hide a problem.
Drilling isn't about airflow. Simple physics more mass can absorb more energy (heat). Drilling removes mass therefore they must get hotter for the same braking force.
Historically the holes were there to stop gas build up that was release from racing pads. Without the holes then the pads would...
Agreed very easy, depending on what you have not it oculd be extremely simple. I done the CD with bluetooth and it just swaps over, an extra cable for the mic (cheap from nissan!), extra GPS antenna, and if you want another cable for the USB socket.