Q1: Yes, just plug the 2nd stepper into any unused driver. Making sure you assign that driver to the Z axis, just like you have done in your config file. I cannot comment on others, but on Duet WiFi, make sure you put the jumpers back on the 2nd Z port.
Q2: Your config file is correct, however, your bed.g isn't. Since you are using 2 lead screws, your S# should be S2.
After you fix the bed, run G32, which will level the Z axis like you want. Run it again, and see if the difference gets bigger. If it does, you need to switch your M671 locations. So instead of X-15:215 put X215:-15 and swap the Y also.
Q3: I run M32 in my homez and homeall Z sections. Do this before setting the home location, that way the adjustment doesn't affect your Z home location.