Just an addition to the discussion on this accident. I have pushed hundreds of aircraft off the gate (D2) where this incident occurred. Never once did I do so without a Wing Walker. I've never seen a baggage handler use one.
The radio clearance is usually something like - clear to push when the Jetstar 320 has passed. It's up to the bloke on the headset to determine when that has occurred and he does not listen into the radio traffic as his mic is direct to the cockpit. There may be a holding short call from the parking aircraft that is heard by the tug driver but unless explicitly told to hold, the outgoing aircraft is free to continue.