I'd say a cargo hold battery fire can be ruled out based on experience. First, the distribution of wiring and antennae means that a fire that took the transponder and VHF Comms would have triggered fire alarms and the halon suppression system. They would have time for a Mayday call, based on the UPS Dubai crash.

Second, any fire that large would have brought it down quickly, so the wreckage would be close to KL, not 6-7 hours flying time away. And a fire serious to wipe out the transponder and comms would have eventually, in not too long a time, wiped out the flight control computers. See UPS again.

I like the story in the Chinese HKG press that the pilot took the plane hostage trying to get the release of Anwar Ibrahim, but that idea can quickly be shot down. The Malaysian government couldn't be that stupid and corrupt or could they?

GF