Even for an experienced designer with almost 15 years of UI design practice, what a great article! You covered great points and made it as clear as water.
I would just be a little careful about the ‘light comes from the sky’ rule, especially when applied to backgrounds. On this very specific case, I think the black focus works well on almost every ‘corner’ of the image. Not a big deal. Just one case where I think the rule may not be so rigid.
Despite that, GREAT reading. Thanks for sharing!