orca, thank you for such an excellent explanation.
Your comment about an 'exponential' rise in workloads when numerous aircraft are in a holding pattern approaching BNE, SYD or MEL is something we should all bear in mind when we are on one of the affected planes.
QF seems to publicise its amended departure times, even if it is 1901 hours ex MEL being substituted for the originally timetabled 1900 hours. However as you imply, there's no guarantee that the flight will take off at 1901, so while the rescheduling is great in theory, in practice it may not always occur as planned. Other domestic airlines do not show these amended departure times on public Internet sites as far as I can establish.
I notice with ATC how some pilots' voices do not carry as well as others. Is this a particular problem with lighter female voices? I assume that ATC has as clear reception and transmission as is technically possible, but when listening to the feeds available through the Internet, some of the less strong voices are difficult to comprehend. Having to twice repeat instructions given by ATC ties up the spectrum for a couple of seconds.