1) Lithium batts may have burned thru transponder feed and created toxic gas before burning out/being extinguished,and the pilot was trying to head back to KL before being overcome by gas.
2) IF there had been that amount of gold on board and there was a suspicion that it'd been hijacked then we WOULD NOT have all those ships/subs in the southern ocean - the authorities would have come out and laid their cards on the table from the start