I am new to this forum since today (15.1.2021) and I have seen your question below. Did you solve the topic already?
Coefficient "c2" is for single-bit, what you have, usually also named as "quantizer" gain (more precisely the "quantizer-dac" gain since "gain" is unit-less", often referred as to "kq" or "k", check Schreier). And this also has to be modelled if you want a correct model e.g. in Simulink. How to calculate it: Check Schreier, Yellow Book, p. 144.
Of course a different kq will shift the STF, too. Make a simple Matlab feedback model, so do a la:
STF = Forward / ( 1 + Forward * Return )
whereas the "Forward" includes the multiplication of "kq".
The full-scale, input referred, is also what is the full-scale of the DAC multiplied by any gain blocks. E.g. if the gain blocks from the feedback (=DAC) are 1 and also the input gain blocks are 1, and the DAC Full-Scale (FS) is e.g. 3V, then also your input-referred FS is _in the average_ also 3V. If there is an additional gain block from the feedback, e.g. factor 2, then your input-referred FS is 6V.