Lightroom :: How To Combine Different Saturation Sliders

I'm not sure I understand how LR4 combines values from the global saturation slider and local adjustments. Two questions in bold at the bottom. I made the following experiments:

1) start with a color raw image, process version 2012, untouched.

2) bring down global saturation to -100

3) add a graduated filter with saturation +100 (I added, say, sharpness -100 to see precisely to which side the filter is being applied)

I would expect half of the picture to be colored and half b&w. Instead the filter is totally ineffective (i.e. it produces absolutely no effect, you can see this rotating it). So I conjecture that the total saturation is not "additive" (i.e. -100+100) but maybe "compound" (i.e. the product of 1+p/100). if this were true, then any slider equal to -100 would bring saturation to the minimum. let's see if this holds true:

4) set global saturation to -100. any value of the filter saturation is ineffective

5) set filter saturation to -100 and global saturation to 0. now (say) in my photo the sky is blue on the left and grey on the right.

6) slowly move global saturation from -50 to +100: the grey sky changes! from a... less saturated grey to a sort of... super-saturated grey (more brilliant and slightly yellow?).

So the conjecture is false and it looks like the filter is applied first, and the global value is applied last, IGNORING what the filter possibly did (so if the filter turned blue into grey, then "saturate" the grey).

In particular, if I want to desaturate everything, except for a very small area, I have to paint the complement of the area with saturation -100 which feels quite unnatural (I'd like to do it painting something only on the small area).