Troy Kerr wrote, "... I completed my 30 seconds of work with the full back lever. Today my elbows are still sore ..."
Excessively sore elbows are not acceptable and you body telling you that you are doing too much. If you continue pushing past your elbows' recovery ability, you are going to give yourself an impressive case of elbow tendonitis. You will need to either reduce the intensity of your movement (e.g. choose an easier BL variation) or reduce the volume of work you are performing (time under tension).
Also increasing 5 seconds per week is too aggressive a progression.
Yours in Fitness,