Hi Graham, I agree that intent does influence folk song singing. And this in turn affects the style. For example, if Frank Sinatra chooses to sing a folk song in his inimitable manner, the folk song remains a folk song but the singer's style is antithetical to it's application in the tradition of the music. Nelson Riddle's orchestrations are not specifically related to the musical style of the tradition.
Sandy, this illustrates the point perfectly. The intent of the young lady singer/songwriter was to write what she thought was a folk song. That intent, however, influenced her singing style. Since she didn't know about the tradition of folk music, her style was inappropriate for the content of the music. There are those who might enjoy a rock and roll version of Mozart. I might even like it for fun. But I don't confuse rock and roll with Mozart even if I thought it was a fun idea. Musical style dictates a different approach, an acknowledgement of the tradition of the music otherwise known as folk music. If a performer wants to do something else with it, that's fine.