Series voltage regulator op amp
Unclear what you mean by &quot;extrema&quot; so I'll answer this:

I also hope to know what these differences affects the regulator briefly.
the direction the diode

Usually a zener diode would be used as reference, so it would be in the other direction. If you use a standard diode you'll get a 0.6V voltage reference. Since R3A=R3B you'd get 1.2V output.

Use pnp instead of npn, reference connected to the inverting input rather than the non-inverting input.

As shown on the schematic the circuit stability depends on load impedance. Open loop gain is the product of:

R3B-R3A voltage divider ratio
opamp open loop gain
1/R2
transistor current gain
load impedance

Open loop phase has a 90° shift due to the opamp, some shift due to the transistor current gain, and of course load impedance. If the load is a cap, that's another 90°.
So the circuit will be unstable on capacitive loads due to 180° phase shift, and conditionally stable on resistive loads depending on what the load value does to the open loop gain, plus transistor behavior vs frequency, etc. Since a voltage regulator is supposed to work on a wide range of loads, it is safe to say this circuit isn't a very useful voltage regulator.
If you want to do a LDO, therefore a PNP-based circuit, then the 90° phase shift of the output cap will always make the 90° phase shift of the opamp a problem. This leads to stability dependent on output capacitor ESR and value, among other things.