Hi @Nikoh77 as long as your VGA to Scart has a USB cable for blanking it will be fine. Something like the following https://www.arcadexpress.com/arcadevga/210-cable-arcade-vga-scart-usb-5v-12v-con-audio.html
When using Linux all resolutions are created for you on-the-fly there is no need to do anything except change the settings in RetroArch. You’ll need to make sure duel display is setup and working properly, make sure you choose the correct Monitor index. Otherwise both or the wrong monitor will change.
From my experience the best desktop environment to use is LXDE. This can be installed on any Linux distro.