D2 is there to ensure current flow goes one way, just in case some accidental connection were to occur.
C6 is not absolutely necessary. It's also there in case of a rogue spike, etc.
In fact the entire network around the PNP transistor 'Q' appears extraneous. It's configured as a constant current source delivered to supply VDD to the IC. (The resistor values are not critical.)
However notice the 15V zener (D7) at the bottom. It regulates voltage so the IC gets a constant 15V.
In this usage a zener can absorb slightly greater or lesser amount of current going through it. Therefore a simple resistor should be sufficient rather than the PNP and associated bias network.
R3 is 1000 ohms. That is probably satisfactory so as to provide enough current to both the IC and the 15V zener.
However it's not clear what voltage the power supply is providing. If it's 120 VDC then R3 and D7 will need higher watt ratings to suit.
It may also be better to use a higher value at C2 and/or C6 so that the IC gets a smooth filtered DC supply.
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Edited to add:
I misspoke. Looks like R3 will need to be higher, more like 10K, so as to incorporate R4 as well.
And the power supply appears to do away with a transformer. This could allow fragile components to be exposed to lethal voltage someday if a short happens in the power supply.