TWO QUESTIONS:
1- How do I identify the "Sub Armature" shown in green in a Python Script?
2- How do I rename that same item?
This isn't a bone, nor is it the Armature (that's "Korra_Pose" at the top of the image.) It's an invisible or assumed root of the armature, and I don't know how to select or identify it separately from the Armature object.