Seems to me that a definition of the ballad is connected to the circumstances. The Ballad sessions at Whitby and Sidmouth this year used a 'working' definition of 'a song that could act as a screenplay to a movie'. This resulted in these sessions having a generally high number of Child ballads although contributors also sang broadside material. This decision making seems to have been in response to the working definition.
Now my personal viewpoint is that a classic ballad is structured using formulaic patterns and language. They seem to be journalistic and, far from merely 'telling a story' they involve complex muli-layered plots. The linguistic devices used suggest that many were originally oral compositions. The great classic ballads have structures which suggest a sense of 'school' of writing/composition. Broadside ballads lack these structures and linguistic devices and reflect more of the individual writers personality than the more formulaic 'big ballads'.
Coleridge's 'Ancient Mariner' is a poem which does not use ballad structures or conventions despite it's 'ballad-like' rhyming pattern.
All of this is only useful in an academic analysis, in a ballad session, the working definition above makes more sense.