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?
I think you’re wrong, you’re not going to change my mind.