The 8.5V regulator around Q1 provides the feedback circuit with a regulated and relatively clean supply voltage. Strictly it shouldn't be necessary, but maybe without it there was too much switching frequency ripple making the optocoupler transmit a pulsing signal that caused instability in the regulation loop...
The circuit around D21 turns on the optocoupler when the output voltage is rising, just after switch-on. So this provides a soft-start function. Once the power supply is up and running, it has no further effect.
And the circuit around Qsc provides slope compensation, required for loop stability in this kind of circuit, whenever the duty cycle can exceed 50%. Google for slope compensation to understand this matter. It's interesting.
Manfred