How to calculate error amplifier output in amplifiers
Ignoring the voltage gain of the Half bridge and just making a feedback loop of the filters;

I get a feedback loop with a fixed low gain &lt;1 proportional to the 500kHz carrier with k1 from R ratios and T1,2,3 from RC products.



I did not bother with the equations but rather direct into simulation with a slider for signal frequency. 

e.g. above signal levels have input carrier Vc ~ 10Vp and output PWM = 15Vp with analog signal on 2nd stage as "output= ~4*Vc" so a net gain of 0.4 Vc.
using 15V PWM supply peak.