I see no problem at all reading the schematic nor with the top connection to D6, it is simply an easy way to add the zener to the schematic without redrawing the schematic because of the tall size of the vertically placed zener schematic symbol.
I don't think you can ignore the time-constants here in charing the 220uF to 12V, they serve a phase delay purpose in the starting the motor.
The two 3.3k resistors have to be so large in resistance to limit the power delivered to the zener diode, but also create a RC delay. They are not limiting inrush current, small-valued resistors always do that and this is not their purpose, their use is in setting the maximum current and power to be dissipated by the zener. If R7 R8 were small enough in value to limit inrush current this circuit as drawn would attempt to use several tens of watts and magic smoke would come forth!
The purpose of R6 is to discharge the cap across it and cause the 2N4403 to turn on during the negative half-cycle of the sine waye AC and change the phase-angle of the opto-triac as the voltage of the zener attempt to rise to 12V. The peak current in R6 is already limited by the zener diode voltage and not at all a consideration here, this is certainly not the purpose of R8 R7.
Consider the original post's remarks about what this circuit replaces, the centrifugal start switches on a motor whose purpose was to switch off the start capacitor circuit when the motor reaches running speed, and this takes some time.
While it is a waste of time to over-analyze a circuit, it is also a fail to rush to conclusions. even a motor-starting circuit knows this.
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