@ Jag. I think I agree with your suggested extension of Harker's definition. Because if the mediator comes between two things then you need the extension to provide the 2nd thing.

Regarding Steve's point: I think you could perhaps say, as Steve does, that a singer mediates a song, but perhaps this only happens when they perform it, or try it out while learning it in the presence of somebody else, as again, there is nothing to create the sense of 'in betweenness'. Does this makes sense?