> 1. Is unlikely as door is very secure now. Passengers can't get in.
The passengers and aircrew would have had seven solid hours to try to break through that door, though, unless the plane landed somewhere first. Alternatively, hijackers (if hijackers there were) might have damaged the door or disabled its locking mechanism on their own way into the cockpit. Some desperate passengers might even deliberately start a fire in the fuselage, I suppose, as dangerous as that would be.
One big problem with 4 is that to believe this you probably have to discount the primary radar reports of the airplane heading west over Malaysia/Thailand and then north up the Malacca Strait and Andaman Sea. It's hard to imagine the plane getting feet-dry on that northern SATCOM arc, at about the end of its fuel endurance, and then then heading far out to sea again. (Unless it landed and refuelled somewhere...)
(EDIT: On second thoughts, I suppose you could turn east after the last radar contact and then go south, past the west of Indonesia, to the southern arc.)
But even if you do that, where would you be headed if you were on that southern Indian Ocean arc after 7 hours in flight? It's not exactly an efficient route to Australia or Indonesia, and everywhere else is either hopelessly far or in the wrong direction.