Hi Thonord
I am going to take a different angle on this then provide some information on Saturation Masks
Here is the analogy. When cooking, when you add too much salt, many times it is hard to correct the problem with other means (e.g adding sugar)
I think part of the issue is that things went out of whack using the Lab techniques and then trying to recover from that step. Lab adjustments are very sensitive and are hard to fine tune to what you want.
I suggest you consider when bringing in a raw file to use either Lightroom or Adobe ACR for the initial adjustments. Alternately, you can make the image a Smart Object and use the Camera Raw filter. The image below was started with the raw image (actually in JPEG) and just using the Camera Raw filter with no other PS adjustments just to give an idea of what can be done. The ACR (or LR) adjustments are easy and intuitive:
That would be my strongest recommendation as the end is an image you want, not the "how" of getting there such as with the use of Saturation Masks.
With that out of the way (all my opinion of course), I think you have seen that there are many interpretations surrounding "saturation" and therefore how to incorporate a saturation mask.
I prefer the definition along the lines is the more colorful it is the more saturated it is. That means that as your approach blacks or whites there is less saturation. PS itself uses several types of saturation. One is HSB in the color picker and another is what is used in the Saturation Blend.
Mathematically, Saturation of the Saturation Blend mode is for any given pixel Saturation = Maximum (R,G,B) - Minimum (R,G,B). A saturation mask could be made knowing just that formula if one knows how to do that in PS. Turns out that just using the Saturation Blend is insufficient by itself (I won't go into details here).
Attached in this post is an Action Set containing one 32 step action that will create this Saturation Layer.
Following is an example test image of all colors at all values of luminosity. This is followed by the Saturation Layer created by the Action. This is basically the same type of Saturation Mask that revnart provided as one of the first posts to this thread:
Note that the Saturation Layer is brightest in the same spots that have the deepest colors and darker when the colors fade away to dark or light. That is my preferred interpretation of Saturation of making adjustments in PS.
Now the Action does not turn the Saturation Layer into an actual Layer Mask though that could easily be added. Just click Cmd/Ctrl + click on the RGB composite channel in the Channels Panel which creates a selection and then convert the selection into a Layer Mask (this will automatically done by adding an Adjustment Layer such as Hue/Sat Adjustment Layer).
For your efforts to adjust saturation of different colors selectively, all you have to do is from within the Hue/Sat Adjustment Layer, change the drop down from Master to the desired color and change the Saturation (and optionally the Lightness and Hue). You can do this independently for other colors by selecting another Color in the drop down list and making additional independent adjustments for those colors. With the Saturation Layer Mask attached, the adjustments will impact the highly saturated colors the most.
Again, I would approach it by not taking the Lab process first yet wanted to be complete in my answer.
Hope this post was of some help (though I was late to the game).