1) Only Field Group with the name "DropDown" is used, if not found, then automatically the primary key and description fields will be used (don't know exactly how the description field is identified)
2) Name the non-default as "DropDown" and rename the existing in "DropDownOld", for example.
3) Every field you need to sort by have to be part of a key (primary or secondary) so, you need to create a new key including them.
4) It's necessary to write some custom code in field's OnValidate trigger.