Is there anything that could have happened that could have "intoxicated" the pilots, crew and passengers, along the lines of nitrogen narcosis?
Not unless someone intentionally put something into the pressurization system. Even then, the air for the passengers is bled from the engines so it is not coming from tanks. You'd run out of agent fairly quickly as the air is circulated out. (Yes, I know the air is recirculated, but only some of it. The rest is expelled from the aircraft to prevent a build up of CO2.)
If something was burning on board (let's just say the already well discussed lithium batteries), it would have destroyed the aircraft long before fuel starvation.
Right now the most likely cause is a deliberate human act, either by the pilot(s) or a non-crew hijacker(s).
At this time, I think it was the captain. He hated the government of Malaysia and decided to cause them as much harm as he possibly could. Figuring that a suicide run with the aircraft would likely fail (it would get shot down before reaching its target or the passengers would realize what was going on and revolt) he decides to fly it as far away as possible so it will not be found, or if it is, it will take ages. (All he would have to do is tell the passengers they were stuck in a holding pattern due to something like weather or an emergency at the destination airport. Few if anyone on board might realize what was happening.) This would be a huge black eye for the Malaysian government. Remember, Malaysia Airlines is a flag carrier, wholly owned by the Malaysian government.