In my eyes, the underlined string above is a contradictio in terminis (oxymoron), as the (step, ramp) input is given to the circuit (CMFB+main), so it can't be a step response and linearity simulations of CMFB circuit alone, as these 2 circuits cooperate in a closed loop.
Of course you can plot/measure the effect of the input stimulation at the CMFB output, but it doesn't say anything about the linearity of the CMFB itself nor of the total system.
So tell us, why would you want to measure the linearity of the CMFB itself?