All of the answers above are true demonstrations of the usefulness of a spacetime (or Minkowski) diagram.
Answer to Question 2
Einstein's special theory of relativity implies that moving objects experience time dilation. The half-life of stationary muons is a known value, and the explanation for greater numbers than classically predicted lower in the atmosphere comes from using the distance covered by the muons and the stationary half-life. The correction comes from the time dilation the muons experience.