Atelier shareholder spotted. But seriously, consider the article.
Addressing your last two points, the reason Vilppu says to analyze but not copy, and Krenz says to reach 80% accuracy, and Finch tells you to copy bridgman twice (from observation then memory, and definitely not using sight size), and /ic/ tells you to be able to draw boxes at any angle is that these methods have you learn how to draw naturally. The notion is to approach a level of skill where you can apply your fundies: anatomy, proportions, perspective using the knowledge you've learned while relying less on guides, measurements, and reference. If you follow the atelier method of using exclusively sight-size then you become le kangourou a la proko, and picture related. Like the article says, the old masters drew forms with geometry and perspective in mind, not by copying shapes of light and shadow. This is the /ic/ anthology. Correct me if I'm wrong