I think you’re wrong about the reference to Downer. They were letting it be known that he is not an employee of the Australian government anymore thus he could not be compelled by Australia to cooperate.
They CAN compel him to cooperate, including forcing him to turn over all communications and all contacts in the period in question to the Australian Government. The Australian Government has authority over their subjects to investigate if a crime was committed, and to prosecute if confirmed. But that would have to be the Aussies, not the USA. And I am inferring from this memo that they will not support that, effectively saying Downer is off limits. And why should they investigate themselves?
There was a recent Australian election.
After an election, the “players” change.
#27. Would be interesting to know if Australian law allows them to bring him before a grand jury there or in the US for official misconduct charges while he was a govt official/ambassador?
That might be a way to get him extradited to the U.S. to testify before a grand jury.