Flyback SMPS design understanding
When powering up the circuit, all you'll have is the main bus voltage, and no oscillations. The resistors connected directly to the bus allow for a startup voltage, to initialize the IC and command the MOSFET with pulses. After that hapens, the switchings will cause the auxiliary winding to provide a voltage, which will take over; that will make the circuit self-sustained. If you look closely, the two resistors are even named Rstartup, and they're two to provide better power handling.
It provides slope compensation. In short, there can be subharmonics when the duty cycle is around or greater than 0.5,  and adding a reversed slope than that of the current can cure it. Since the current is trapezoidal or triangular, the ramp oscillator provides a readily available resource (the base of Q1 is at the RT/CT pin). The naming of the transistor and the resistor have sc in it, which probably comes from &quot;slope compensation&quot;. There are plenty of links if you search the web, or see this answer, for example.
The bus voltage, for a 230 Vrms from the mains, will be 325 Vdc. The inductive switching regime of the circuit will cause the emf to reach twice that voltage. If you account for a maximum RMS input of (a fairly common) 265 Vrms, the peak would be 375 Vdc, times 2 that's 750 Vdc, add some clearance (maximum datasheet value, spikes, variations, etc), and you get 800 Vdc.
The output is either high current, or it uses a two diode package (looks like a transistor). If it's the first, it halves the dissipated power on each diode, if the latter, you can't really cut in two the package (and expect it to work), and it wouldn't make to much sense to use only one half (why did you bother to buy it in the first place).