Hello, a first-time poster here with a couple of points I would like to contribute:
When Tim Clark talks about 3 systems disabled, he probably refers to the transponder, ACARS and SATCOM. Please note that ACARS and SATCOM are separate systems.
Transponder being disabled (turned off or failed) is self-evident.
Last ACARS transmission from the plane through SATCOM was at 1707 Z and none received after that, thus ACARS disabled.
In the Inmarsat logs, there is a gap of SATCOM transmissions between 1707 Z (after the last ACARS transmission) and 1825 Z (when the SATCOM in the plane initiated a handshake). In particular, the SATCOM modem didn't respond to a satellite query at 1803 Z. We can deduce therefore that SATCOM was disabled at least between 1803 Z and 1825 Z, and enabled at 1825 Z.