So if the computer can still operate the mouse position (in a crashed state) why then cannot it (be made to) operate any other emergency kill process functions?
Quite possibly there may be a crash/loop in keyboard handling code but the mouse handler code still works - as seems to happen here.
Have you tried ctrl+alt+backspace to kill X? If this fails then try the "raising elephants" trick - i.e. hold down Ctr+printscreen and hit (slowly in turn) R,E,I,S,U,B - wait a little while for a result after each alpha key is hit. This nearly always gets through and will shut down stuff in a more orderly manner than just powering off or rebooting.
Edit: Hope you see this edit - I got the above a bit wrong it's Alt+printscreen and hit (slowly in turn) R,E,I,S,U,B - sorry - I don't often have to do this!