To your questions:
a) Yes, 12V / 2A is ok.
b) SW1 = 0 / SW2 = 1 / SW3 = 0 / Each motor coil is set to 0.5 A (I had changed it in the meantime). You can increase the current to 1A. Then SW1 = 0 / SW2 = 0 / SW3 = 1.
S1 = 1 / S2 = 0 / Decay mode = 25%. Find yourself an optimal setting to reduce the motor noise at standstill.
S3 = 1 / S4 = 0 / ½ step mode. You can also choose 1/8 step mode.
S5 = 1 / S6 = 1 / Torque = 20%. Choose an optimal setting for a „smooth“ run.
c) I had glued the 3 legs of the (black) mini trim potentiometer directly on the backside of the LCD display. It is clearly visible. It's not necessary to adjust it from the outside, only 1x and that's for ever.