Vcm = (Va + Vb)/2
Vdiff = Va - Vb
So how observe Vcm if you only have one signal level, e.g. Va?
In so far the answers to question 1 and 3 are obvious.
Only question 2 isn't self-evident from the start. The answer depends on 1. circuit details, 2. circuit purpose, 3. operation conditions.
CMFB may be required either because you want a specific Vcm level at a differential amplifier output or because the circuit would saturate (not work as amplifier at all) without it. Decide what's your requirement and analyze the circuit of interest.