What is ACARS using to report status? Is it using SATCOM? If so, couldn’t it be disabled by a hostile crew? It’s farfetched but possible. As for AHM, are you saying it pings the Boeing servers even if the country isn’t a subscriber? Nevertheless, an Iridium transceiver could be disabled, too. Again, it would take a knowledgeable crew to do something like that, but I think that’s where the conspiracy theories are leading. If a terrorist crew was smart enough to take over the flight and fly it to a secret destination, they’d probably know about the on board systems.
The Iridium transponder was not disabled. That was the source of the pings. The satellite cannot be controlled or disabled by a user. The ACARS would only tell us that the gear came down.
Anything it can. SATCOM overwater, with HF secondary backup, again overwater, out of VHF range.
Simple to disable through circuit breakers in the cockpit.
AHM is a different system completely.