If you apply system knowledge you could say that the DRIVE failure is the cause of the SOURCE OFF annunciation, so it would make sense to go to the DRIVE NNC first. However, nowhere in the QRH does it say that the DRIVE NNC has a higher priority than the SOURCE OFF NNC, so it would not be wrong to look at that one first.
Yes, the SOURCE OFF light can come on without the DRIVE light. SOURCE OFF doesn't mean that the IDG has failed, it only indicates that the last selected source is no longer powering the associated transfer bus. You might or might not be able to select it back on the bus. However, ask yourself why it tripped off in the first place and whether it could happen again.
Ps: unless your company uses a modified version of the QRH there are no memory items associated with either checklist. Either NNC will direct you to start the APU, it is not a memory item.