There is not usually much of a queue at the Disney Car Care depot (airports are usually worse as large number of people arrive at the same time based on flight arrivals).

All the car rental companies generally have very dubious car seats as they are well worn (although should meet safety standards). You will always have to fit them yourself with little or no direction from the staff, as they don't want to risk any liability if you are involved in any sort of accident and a child is hurt. If they have fited the seat there is a risk that customers could point the finger at the rental company.