These seem to be contradictions.
If pixel values are proportional to intensity, then multiplying (or dividing) all pixels values by the same number will preserve that proportionality.
But adding (or subtracting) a value removes that proportionality.
For a more technical perspective: multiplying RGB values by the same number preserves chromaticity (x and y channels of xyY, which, rougly speaking, preserves hue and saturation). But adding the same number to RGB values changes chromaticity.
Subtracting a black offset from pixels may be needed to make pixels linear, ie proportional to intensity. Once that is done, any further addition or subtraction will remove proportionality.