The file list on the right is ordered exactly like the file list in the Mp3tag main window. This can't be changed by the script. But you can change it easily in the Mp3tag main window before using the script. To sort by discnumber, you may have to add that collumn if you haven't yet:
from the FAQs:
You can also add additional file list columns for extended tag fields via View > Customize Columns... and edit the tag fields directly in the file list.