At this point I would consider this as a Nat. Security matter.
IMHO the plane is on the ground somewhere intact until it can be proven it
crashed. If it crashed at sea what are the odds it sank in one piece ie no debris.
If it was landed at a remote spot it has been hidden by now.
What type of tech would be required to see a covered or hidden aircraft?
Would I.R. work, Sat. based Radar?
Considering our history with Terrorism and Aircraft IMO this needs to be
classed as a Severe National Security Threat !
What is the down side for anyone IF the USA treats it as a threat?
I don't really see any, the plane gets found, good or bad the families get closure, the threat is rendered harmless.
I do not see any downside for the Malay govt. either, they have already lost "Face", unless perhaps they are complicit in the "Loss".