- Using the curve, what is the blending mode ? Normal ? Soft Light ?

- Using desaturation and high contrast curve to find imperfection, to desaturate, do you use hue/saturation layer adjustement and move the saturation slider all the way to the left in Master ?

Whether to use a blend mode other than Normal depends on what you're trying to accomplish.

The 'contrast enhancing' modes of Overlay/Soft Light/Hard Light can increase contrast even without any Curve changes, or reduce it by inverting the curve. That can additionally be tweaked by actually modifying the curves.

Screen and Multiply are useful for lightening or darkening an image overall, as well as enhancing curves manipulation which tweaks those aspects.

It Depends :)

Using Curves almost always means leaving the mode in Normal.

Any curve adjustment set to Normal can produce precisely the same result that a curve set to a blend mode can do, you just need to know what the points are t use to simulate each blend mode. So there's a mistake in assuming that the blend modes offer anything new, they don't, only make things a little easier.

In I think its true to say that if you make adjustments to a curve and then also apply a blend mode to the same curve you are applying two rounds of correction to the image. So its more destructive that way. Least thats my understanding of how the adjustments are applied, in sequence

Using Curves almost always means leaving the mode in Normal.
That is vastly overstating things. Probably 30% of my Curves layers are in a blending mode other than Normal--Multiply, Screen, and Overlay being (by far) the most common. Taking advantage of blending modes lets you use Curves with more control and/or more brute force.