Why is the sky blue, revisited

Factual Questions
1

Inspired by this xkcd: xkcd: Sky Color

Cecil coloumn: Why is the sky blue? - The Straight Dope

So, the sky is blue because that the light is scattered around by air particles. The short wavelength component of sunlight is scattered more than the other wavelengths. Blue has shorter wavelength than red and yellow.

But violet has a shorter wavelength than blue. So if this is the explanation, why is the sky not violet?

Proposed explanations I have encountered:

  1. Because the ozone layer blocks out the violet light more than the blue.
    (Thus, even though the violet light is scattered more than blue light, there is much less of it to begin, so the final impact is small.)

  2. Because the sun emits less light in the violet spectrum.

  3. Because the wavelength - scattering relationship falls apart in shorter wavelengths. Violet light does not actually scatter more than blue light.

  4. Because the way we humans perceive light with the cones and such.

  5. Because that mixture of violet, blue, red and yellow is blue. (Not sure if this is distinct from 4)

  6. Because blue is the color of Oxygen and Nitrogen is colorless. (As far as I understand, this explanation is false. But the correct reason why the sea is blue is because water is blue.)
    So I have two issues. One: Are these all really factors with an actual importance? And two: Is one of them sufficient by itself to make the sky blue, or are we sure that this is not the case?