Yes, it’s possible. You have to set fade power to zero, and set “Selective recolor” to Luminosity_coloring, with color set to gray. You can also set Recolor power in an interval between 0 and 1, depending on how much of desaturation you want.
Yes, Advanced Selective Coloring is an image effect. If you want shaders that do similar things, you can look at Colorify.
Only if you use pre-baked variants. Most processing-heavy operations in both Selective Coloring and Colorify happens to be color comparison - recolor and fade in itself are very fast operations.
Also note that Colorify doesn’t include image effects - it only includes shaders that work as an alternative to standard Unity ones.
