I've just tried and the option to not show the keyboard actually 
works fine (no matter whether the desktop is localized in German or 
English).

It's exact name in English is

"Show virtual keyboard on activation of Behaviour"

and you can find it by:

1) Opening the prefs 
2) Clicking on "Behaviours" 
3) Selecting "Virtual Keyboard" from the list on the left 
4) Selecting the "Options" tab 
5) Then, turn off the checkbox in front of "Show virtual keyboard on 
activation of Behaviour"

From that point on, when you activate the Behaviour via the menu, 
the virtual on screen keyboard should no longer show up.

You could "combine" several modifiers by subsequently pressing 
different buttons - yes. The modifiers will not reset automatically, 
though, but only as you press the buttons again. Let me know, if you 
need an action that resets the modifiers with a single press - it's 
not hard to implement.

Best regards, 
Felix