Ian W,
Now as also said you have to come up with a practical way that a fire in the hold that would immediately trigger an alarm in the cockpit, could stop ACARS, SSR transponder and the three independent VHF radios in the short period of a simple change of frequency usually by selecting the 'other box' - without any distress call from the crew.
You are quite right About 3 different VHF sets and obviously there are 3 different antennas for these radios and different power Buses aswell. But how are the coaxial connections and coax run through the airframe to these antennas? That's what we don't know unless we have some B777 Avionics guys out there to tell us. If there are wire bundles or coax bundles, that could have been severed simultaneously because they were collocated .If that's been discussed and answered before please forgive me.
I agree with your "Lithium Ion phobia"synopsis. We have been flying many Lion batts for years. And it will only increase with the proliferation of Lithium Ion power sources and applications. That's why rules of carriage for the items need constant adjustment as new technologies are used. It needs to be proactive rather than reactive.