The Panasonic System 3000i IFE fitted to 9M-MRO did not have cabin voice telephony capability.
It did have the capability but it was disabled to the passenger seats. That does not mean that the SATCOM call capability from the Cabin Managers station was disabled. Qantas use a similar system and have the ability to make a SATCOM call from the CSM's station even though the seat capability is similarly disabled, even with the IFE switched off. In all probability so did MAS.
Re: the Oxygen. All the oxygen in the world will be useless at 35,000 feet if it is not delivered at a substantial pressure.
The oxygen generators supplying the drop down masks deliver 3.8 litres per minute for 7 minutes, after which the flow rate drops to 1.8 litres per minute till exhausted. These flow rates are not enough to maintain consciousness for any length of time at 35,000 feet.
In contrast the Flight deck system is required to deliver a minimum of 20 litres per minute, delivered at 100% with an emergency pressure if required. The cabin bottles have a maximum of 4 litres/min capacity, also not delivered with sufficient pressure to keep someone cogent and functioning.
Yes the Helios crew member survived at 34,000 feet but was unable to operate the radios despite being a CPL holder. Clearly the oxygen he was receiving was enough to keep him semi-conscious but he was he was not functioning. No-one else on the flight was able to remain conscious.
For fear of stating the blindingly obvious, the search of the area at the end of that straight line turned up bupkis.
The ATSB, for some unknown reason, models the "End of Flight" scenario on no-one being alive on the flight deck. If someone was, someone with the ability to balance the fuel difference and fly the aircraft in any manner they chose after both engines flamed out, then the area required to be searched widens considerably.
They chose that area for good reasons and discounted the curved flight paths. The straight flight path can only be achieved by someone with systems knowledge and deliberate intent.
However, that technical error in no way impacts the fact that curving flight paths can be excellent fits for both the BTO/BFO data and the fuel flow/exhaustion data
. Not if it went into Speed Mode. The curved flight paths require a reduction in speed. Speed mode holds the current speed at the end of the route.