<i>At what point in the process will I know who the rental is actually with and if there is a fee?</i>

Generally Kemwel uses Europcar in France. However, that applies mainly to train station and airport pick-ups, for which there is a fee, but which is lower than the one way fee charged by Hertz. In town pick-ups (at least in Bordeaux) will be through Hertz or Avis.

The Kemwel agent will know if there is a one-way fee or not.