OK, nice work. Good clear pictures and descriptions and information there.
It looks like pin A connects to a tap part-way along the carbon track in the pot. It would be intended for a "loudness compensation" connection, but as you say, it's not connected to anything on the PCB, so you don't need to worry about it.
The wiper (C) and the two ends of the track (B and D) are clearly visible, and the B end is the fully clockwise (i.e. maximum volume) end, based on your description of which pins the wiper connects to when the pot is fully clockwise and fully anticlockwise.
Looking at the PCB track side, I was expecting to see the anticlockwise track end pin, D, connected to the 0V rail of the circuit.
The 0V ("zero volt") rail is usually easy to recognise because it connects to a whole lot of components, and the outer screen braid of any screened wires that connect to the board, and the 0V rail of the power supply, and sometimes, the PCB mounting screws, and it's usually a thick track with large copper areas on it.
The 0V rail is the "no signal" connection point for the whole circuit, so when the pot is turned to that end, the wiper (which is the "output" of the volume control) is connected to the no-signal end, so the amplfier goes fully silent.
So I was surprised to see that all three pot pins have long thin tracks running to them.
It's possible that the track from pin D connects to the 0V rail through a resistor; this would mean that at the fully anticlockwise position, there is still SOME sound audible from the amplfiier. This is done with some amplifiers; turning the volume control fully anticlockwise does not FULLY kill the signal; it just reduces it very greatly.
Anyway... It seems pretty clear that jumpering pin C (wiper) to pin B (clockwise end) will bypass the volume pot and give you maximum volume.
If there's any doubt about pin D, you can connect a resistor of similar value to the pot, i.e. 20K (the closest standard resistor value is 18k or 22k) from pins C and B to pin D will be exactly equivalent to the pot being present and turned fully clockwise. So you might want to add a resistor on each pot position, as well as jumpering those pins together.
I notice that there's some crud around the holes where you've removed the pot. If you want to tidy it up, you can use a cotton swab (here we call them "cotton buds") with firm pressure to clean the PCB. If you have isopropyl alcohol, or any MILD solvent, wet the cotton swab with that first. If the pads have lifted or become less securely attached to the PCB due to overheating when you removed the pot, it's probably best not to rub them hard though.
Finally, bypassing the volume pot and setting the amp to maximum volume may cause problems with background noise. All the noise (buzz, hum, hiss) from the preamplifier stage (the stage before the volume pot, that connects to pin B) will be fed to the next stage (connected to pin C) at maximum volume, and this may mean you hear too much background noise all the time. You can try it and see.