what is the value of D3 and D4 diode in the sch and whats the use of them?

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#2
I'd be more concerned why there are differences between component values on the schematic and the same components in the parts list.

Are ZD5/6 6.8V or 15V ?

The diodes D3/4 have been added to adjust the zener voltage by 0.7V. I would guess that 1N4148 would be OK there.
 
#3
They are zeners to ensure the gate to source voltage for the FET's isn't exceeded. The voltage is whatever that diode number is, 7.5 volts according to a quick search. Same function as ZD5 and ZD6. Hard to believe the first ones are even needed.
 
#6
Its as KandD says. It adds around 0.7 volts to the zener, but they also lower the reverse leakage dramatically because zeners are rather poor in that respect and they also increase the maximum reverse voltage when the zener would be non conducting.
 
#9
The diode is to allow the Vgs to go into cutoff. If it were not there, then the gate drive would only be able to go into cutoff 0.6V, or the value of a forward bias
Zener, limiting the time it takes to turn off the divice. The regular diode should be fine for such a general driving stage. I typically use two 15V Zeners in series stacked in opposite directions for this reason and to prevent Vgs from exceeding the 20V breakdown in either direction.
 
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