Thanks to Randjgc for a most useful post, reminding us that these commonplaces are part of the descriptive process of ballad creation rather than being specific links in plotting stories.
My own belief is that the 'bones' of a ballad are relationships (mother/son, sweethearts, husband/wife, siblings, employer/servant etc), because that's where dramatic tension lies.
And dramatic tension is surely what the ballads are about.