"Steer zero nine zero", "Zero nine zero aye" is the right way of doing a callback compared to "Ready about!", "Ready!" Repeating the information back, instead of just saying "yes" ensures that the correct message has been received.
My experience with this was relaying a message between the driver and guard, to reverse a train on a main railway line. The guard answered "yes" and there was hell to pay as the train reversed across a section boundary. The "yes" response ensured the guard received a message, but not that he received the correct message.
You are correct. For many commands such as raising sails we would repeat the command across the deck. My recollection was that distinct responses were reserved for potentially dangerous maneuvers where the helm needed to make sure the crew was performing the correct step at the correct time.
My experience with this was relaying a message between the driver and guard, to reverse a train on a main railway line. The guard answered "yes" and there was hell to pay as the train reversed across a section boundary. The "yes" response ensured the guard received a message, but not that he received the correct message.