@IanW
Thank you for the explanations.
What I've taken away from this very interesting discussion is that possible the ground would know if the system had been tidily shut down because a logoff message would have been received, but that the existence of such a message hasn't been explicitly disclosed publicly. As you quite rightly say, if that evidence exists, it is crucial.
The preliminary report simply states:
It was later established that the transmissions from the Aircraft Communication and Reporting System (ACARS) through satellite communication system occurred at regular intervals starting before MH 370 departed Kuala Lumpur, Malaysia at time 12:56:08 MYT and with the last communication occurred at 01:07:49 MYT.
I was a bit surprised to read that paragraph, as I had been under the impression that ACARS was VHF only on this flight. Like others, I've also been under the impression that the 1:07:49 message was a routine engine report.