What IS reflection?

the process is coherent--- the same photon is bouncing off all the atoms at once, and you only get constructive interference when the angle of reflection is equal to the angle of incidence. The condition is that the surface is smooth on the scale of the wavelength of light, so that the light can excite each atom independently, and coherently add up all their contributions. This is Feynman's explanation in QED, I think you just misunderstood it as saying that reflection is assumed--- he just assumed rescattering, and then shows you that it happens in the reflected direction preferentially.