Is it possible to make it show only the desktops on the current monitor (similarly to KDE)?
I've noticed the same issue before, and have spent a long time trying to find a solution.
As I said, I have spent many a long hour studying it, and the only answer I can post, I'm afraid, is no, there is no way to do that with the Expo plugin. It has to be controlled by compiz, and the settings are unfortunately extremely limited. I searched around to see if there would be some way to change configurations, but it looks to me like short of editing the source code and recompiling, it is impossible.
The only workaround I found personally was instead of using the Expo plugin to use the Desktop Cube and Rotate Cube plugins, setting
Multi-Output Mode to
Multiple Cubes and unfolding the cube instead of opening the Expo. I don't like it as well, and it just shows a black or transparent spot where the other screen should be, but that's the best I've found.