thanks for posting this. It's very helpful when one can see the specific steps.
You noted:
Given that, why not do step 2 with a luminosity blend mode? Wouldn't that obviate the need for step 6, unless the original saturation is problematic? I often do curves adjustments with a luminosity blend mode to avoid changing saturation.The problem with RGB colour spaces is that the brightness and saturation are not separate values, so when something gets darkened up, saturation also increases and looks wrong.