Hi!

I have a small netbook that I connect with a monitor (vga-port) when I want to use it for development.

This is the feature-set that I want:
1) When I close the laptop lid, only my monitor works, and works as primary desktop.
2) When monitor is disconnected, my laptop screen works as primary desktop.
3) OPTIONAL: When both screens are available (lid is open and monitor is connected), the monitor/current desktop with mouse should take preference

I have been struggling with system-settings for 3 hours now and even after much googling I have been unable to find the solution for above problems.

Any idea what to do?

This is an area which needs work within KDE unfortunately.
For you needs - you may find running individual scripts to change the X settings (through xrandr) may be best.