Hello, my name is Mark and I will try help you with this.
To replace the heater unit does involve removing the dashboard. The time involved for this is around 8 hours.
Mercedes do only replace a defective component under warranty not the complete assembly.
They would have checked the system at the time but if all the other components were ok at this time then there would be no need to replace them. If the garage had replaced them under warranty when there was nothing wrong with them the claim by the garage to Mercedes would have been rejected.
As a loyal customer I think the best you could get would be a goodwill contribution towards the costs but they are not obliged to do so but there is no harm in asking.