I mentioned the light temperature.thing in detail above, I think the crucial difference if you replace the moon by a mirror is that the mirror would preserve the etendue of the sunlight whereas a scattering surface greatly increases it, and so we lose the ability to concentrate great amounts of intensity by just using an idealized lens.
The fool hath said in his heart, There is a God. They are corrupt, they have done abominable works, there is none that doeth good.
Psalm 14, KJV revised edition