=> I assumed high current and voltage = better effect on the body!
Many people thing this too. You can even find time to time zappers with 3x9V batteries.
Human body is a very complex “Think”, and more is not always better.
My understanding is that 50uA to 150uA TO DESTINATION is enough to make a zapper work.
How much should we inject into the skin to have these numbers is very debatable, but the original Clark’s zapper was working well with 3.5mA peak.
=> Would 9v be just as good as 12v?
It depends on the body’s condition and where you want to push your current, because a zapper is ultimately working with CURRENT, and voltage is there only to push current to flow.
Dr Clark said that 5 to 10V was enough.
There is a small zapper, the MZ5b, working with only 5V. Not always enough for heavy cases, but enough for general use.
There is too some “smart” zappers working with current, instead of voltage, as the MZ3b.
=> You mention a push-pull stage, isn't this used when the voltage goes below zero as in an audio amplifier?
Sometimes, yes, when the load (the speaker) is at mid voltage. Zero voltage reference is very relative.
On your output, the NPN transistor will pull to ground the output signal quite sharply, with close to 0 impedance, and let the load resistor of 100_Ohm go to +12V slowly, because the load (you) is capacitive.
You can easily see this on your scope shot, where the fall time is sharp, and the rise time rounded. Not very important time wise on 30kHz, but more important on lower frequencies, where most people want to add harmonic frequencies to the basic zapper effect.
A Push Pull stage will pull to the ground, and push to the 12V with the same sharp strength
There is a safety issue too : When output impedance is very low, as on fall time on your project, only the resistance of the body will limit the current. Not very safe on every situations.
A 555 has just this stage in its output, is low priced, and needs no other component if used as a buffer. Only the 1k_Ohm output resistor will have to be added.
You can do the same thing with other components, as OpAmps, but the 555 just fits well and has a good band pass frequency for this use.