Questions regarding eliminating hysteresis in ac dimmer
The root cause of the hysteresis (aka "snap-on" effect) in simple triac/diac dimmers is the capacitor not being discharged by the diac in the previous half-cycle and entering the subsequent half-cycle with the opposite voltage across it. 

Consider just the positive case where the capacitor is negatively charged from the previous half cycle. The negative case is just a mirror image. 

Imagine C1 was charged to -10V, and the mains voltage has passed the zero crossing and is heading positive, at the instant when it is +30V. 



The current to charge C1 to approximately 0V must come through R3 from the mains, clamped at +0.7V by D2 and then flow through D1 to charge the capacitor, so the capacitor receives charge as long as its voltage is approximately less than 0V. 

Once the capacitor has charged past 0V, D1 no longer conducts and the capacitor charges through R1+R2 towards the positive mains voltage. D3 and D4 are reverse biased and are not in play in this half-cycle. 

Since R3/R4 are relatively low value the discharge takes place relatively quickly. There is a trade-off between how quickly the capacitors are discharged and the power dissipation in those resistors. 

In the example, there is a couple of mA (depending on the mains voltage and the capacitor voltage) available to charge the capacitor, which means it will only take a couple hundred microseconds to get it close to zero volts, which is quite acceptable.