Couldn't the strange trajectory be explained by damaged, slowly hypoxic, brains trying to find a solution but unable to exercise sufficient logic to save the plane until, finally, at the last turn, coma overcomes the pilots?

Or a la Helios, a FA on O2 Scott bottle. For me this has been the least implausible theory.

I'm not so sure.......although there's no harm in checking the data again.

The washed up wreckage corroborates the Inmarsat data.

No it doesn't work like that.
Useful consciousness is around 90 - 120 seconds at that level. Anyone not on emergency oxygen would have been comatose after the turnback.

It does not explain the loss of those electrical systems that broadcast the position of the aircraft but not others that do not. The only radiating system on the aircraft that stayed on was the SATCOM that most pilots don't realize is continually talking to the INMARSAT satellite, and the one that most pilots would have difficultly knowing how to switch it off. Almost nobody realized that the corrections for doppler shift for the communications channel could be used to identify a 'range ring' and possible maneuver of the aircraft. Only the INMARSAT team realized that. It may be worth checking their maths - again - but that's been done ad infinitum.

The fact remains that the aircraft flew into the Southern Indian Ocean coincidentally :rolleyes: to a point close to where the captain had been practicing using his home flight simulator doing landings on sea level airports where there is no land. We know the aircraft entered the water around that position as bits of the aircraft have washed up in Madagascar and coasts around there at a time that would match with being carried by the ocean currents and with barnacles of the right age for that length of immersion in sea water and of the right type for the likely location +/- 500nm.

What we do not know for sure is why - it may not have been for its cargo of mangosteens.

A little research:

SDU (SATCOM) is powered by Left AC Bus
Other systems powered by this bus are not documented (BDL Bus Distribution List not available for 777) as far as I can see... that is if you lose it that's when you may find out (some items not EICAS messaged on fault)
You can piece together from Maintenance manuals but not in one place.

E/E bay access likely needed for CB, then straightforward, 3 CB's.
Looking at the docs access to the SDU via E11 rack could be achieved in the cabin also (not sure about this as I don't know how easy it would be to access).
Disabling from the Cockpit... unlikely.

Disclaimer: I'm not an Engineer.