The latest is that there was a 14 minute gap between when the ADS was shut down and the transponder, which nukes the total electrical failure theory. Also, it is logical that the sat system would continue pinging regardless whether they were logged onto CPDLC and actually sending data, and even if they de-powered the ADS it is likely the other performance related reporting systems may have been pinging on their own. Don't know whether they are independent but I suspect they are.
As to the decompression theory, I cannot imagine the emergency oxygen system can be disarmed from the cockpit and the flight attendants have their bottles, so one of them should have been able to get to the sat phone. Besides, I don't see the need in darkness over water with a flight expected to last hours... they just keep cruising along, making cabin announcements about the headwinds, etc., until it's time to roll inverted and aim for the water. Alternatively he (or they) might just swallow a bottle of sleeping pills when there is only a half hour of fuel remaining. The objective was to make the plane disappear without a trace, CVR never recovered, and the southern Indian Ocean is the place to do it.