If you aren't doing it already keep a record of the oil you put in, recording the date, the odometer, and the amount of oil added. That would give you some evidence to present to the dealer and MB. As you haven't had any satisfaction from the dealer go directly to Mercedes-Benz (Ph: 1300 730 200). There shouldn't be any reason to be scared on driving the car, all cars use oil, it's the amount of oil used that it the question that needs to be answered in your case. In the meantime check the oil level weekly.

Your frustration is understandable. I would urge you to go back to Mercedes-Benz direct and request some assistance with the cost of repairs. While I doubt that Mercedes would cover the whole cost of the job, I would think they might cover part of the cost if you approach them in a reasonable way. If you don't want to go down that path consult an automatic transmission specialist and get another quote for repairs.