Many interesting versions have been addressed and discussed, but the "Occam's razor" principle suggests to first evaluate and elaborate the most obvious options.
IMHO, this means to focus on a "trivial" hijacking. If that was possible in US back in 2001, why similar guys (e.g. islamic extremists) cannot do that now in KL where the security is not very tough (my personal opinion as a passenger that flew to/from that airport several times in the recent years)?
A long and careful (and hidden) work of many security services may (and hopefully will) bring more on the desk than chaotic search involving many planes and vessels. After 9/11 the details of the entire gang were revealed quite quickly. Thus, it was doable.
It appears that in China (because of their known regime) they could screen background of their passengers, but there were people of many nationalities onboard. I wonder if all the countries whose citizens were in the plane are eager to cooperate and have capabilities to dig out all the aspects of their personal life? But again, this now seems to be the only way to crack the puzzle. If the debris were not found in 10 days, the probability of finding them later reduces exponentially.