Various
Apparently according to MAS here was nothing hazardous in the cargo hold. We have no other information, no cargo manifest, rien, nada!
The ping is initiated by the satellite asking the aircraft if it is still active - the aircraft replies to ping only with the statement 'I am live' however that maybe configured in satellite speak. NO DATA is passed. The satellite continues to ask the aircraft on an hourly basis in order to keep the communication line open 'in case' the aircraft wishes to pass data.
There is no confirmation that the last 'reply' from the aircraft was from the ground; However we do know that if the aircraft had landed and the engines were still powered it would 'reply' with an 'I am live'.
The passengers have not responded because why bother taking mobile phones at gun point when you can just 'dump' the cabin and then take the aircraft up to 45000 feet or so (I say or so because primary radar cannot confirm with the parameters it has it was exactly 45000 feet) to make the process faster!
I think the answer is going to be a combination of several scenarios already mentioned on here - amongst the more literate responses. Which ones who knows - but my personal opinion is Murexway at #5488 was pretty close together with a previous poster who also mentioned cabin dumping and once dumped surely the cockpit is going to get pretty cold! Something not even a SIM can prepare you for!