Well, i have solved the major part of the problems: i bought another keyboard. I got a Logitech G810, connected next to the Dell Multimedia keyboard, and this one worked. I was able to enter BIOS without issues. So, it is now very clear that Asus Maximus X Hero has compatibility issues with a lot of keyboards (i have tested 7 different keyboards, only with 2 of them i was able to enter BIOS).
The other problem, with my USB devices, is strange... and i am trying now to see if this is also a compatibility problem. I will check back in here after some testing, swapping, reinstalling... it is sad to see such a top motherboard acting worst than a cheap one, wasting my time with tests i shouldn't do (Asus engineers should do that instead)...
MonkY, did you enable Fast Boot by any chance?