Once the aircraft was low enough and in range of mobile phone masts then all mobile phones could be tracked and positioned with some degree of accuracy.
There are no towers in the ocean.
The phone does not even have to be on. Just the battery installed.
Wrong, phones off or in airplane mode do not make any network activity whatsoever.
Once a few mobile numbers of the pax have been identified, pretty simple task, then the position of these phones should be easy to locate
So catch a fly to KL, join the investigation, they will tell you the clown you are instead of me.