Alot of Key Commands doesn’t work when MixConsole isn’t in focus. Is there ANY way to make it “in focus” by any key command?
Same goes for Arrangement window…
Control+Tab will cycle focus through all open windows. So if you only have the mixer and arrange window open, this will toggle focus between them.
And for Apple users, hold down the CMD key and push repeatedly the key , to skip through the open windows in Cubase. (The key is on the lower left of your keyboard, above the CTRL and ALT key).
The chosen window by this key combination will be the focussed window. 
Unfortunately, control+tab wont get you to the mixer if you have it full screen.
Hugh