How can I restart Compiz from a TTY? (& Related, how can I set up a fallback WM?)

Switch to terminal number 1 (Ctrl+Alt+F1) and run

DISPLAY=:0 unity --replace 

or

DISPLAY=:0 compiz --replace

Sometimes it doesn't restart. Just kill all compiz processes and try again in case.

You need to set the DISPLAY environment variable, in the usual case of a single Xorg instance you can do that with

export DISPLAY=":0.0"