Answer for the 1st question.
In order to restart your X server with "Ctrl + alt + Backspace",
Open "System Setting > Keyboard Layout".
On Keyboard Layout, click the option button in bottom right corner.
Expand “Key Squence to kill the X server” and then check "Ctrl + alt + Backspace"
Answer for the 2nd question.
Magic SysRq keys active by default in Ubuntu 12.04 LTS.
You can check to execute this command:
sudo cat /proc/sys/kernel/sysrq
Then, you can see "1" in terminal, this means "enable all functions of sysrq".
To perform a safe reboot, use REISUB SysRq Magic combination key:
Hold Alt + Fn + SysRq then type R+E+I+S+U+B , you need to press each key for 2-3 seconds.
Caution: Don't release Alt + Fn + SysRq, when you type R+E+I+S+U+B. And, after you type "E", black screen comes out. But you have to continue typing "I+S+U+B".
So, you can reboot your PC safely.
Safely shutting down Ubuntu when it is frozen, use "R+S+E+I+U+O" keys instead of "R+E+I+S+U+B".
Meaning of each keys:
B: Will immediately reboot the system without syncing or unmounting your disks.
O: Will shut your system off (if configured and supported).
R: Will switch the keyboard from raw mode, the mode used by programs.
S: Will attempt to sync all mounted filesystems.
U: Will attempt to remount all mounted filesystems read-only.
P: Will dump the current registers and flags to your console.
T: Will dump a list of current tasks and their information to your console.
M: Will dump current memory info to your console.
E: Send a SIGTERM to all processes, except for init.
I: Send a SIGKILL to all processes, except for init.
L: Shows a stack backtrace for all active CPUs.
I think that you may arrange the combination of these keys (after considering the meaning of the keys) , since this is an example.
You can reboot system using only B command key, or shutdown using only O command key. But, especially , it may be important to remount a file system safely by "read-only" using U key, to protect that a file breaks in the case of forced termination.