Let's resume another theory.
1. Say one plane followed MH370 either stay directly below or above MH370. In the ATC radar only picked up one plane marker on the screen.
2. At certain time both planes switched transponder signals and MH370 use to the other plane transponder signal and fly on the route to North Korea.
3. The other plane switched to MH370 transponder and continue to fly assuming MH370 plane.
4. These switched over was done earlier and at certain time the other plane switched off the transponder and assumed MH370 went missing.
5 Or there could be more than one planes following MH370. One plane fly parallel with MH370 and MH370 switched off transponder. ATC not suspecting anything went missing as the other plane has transponder signal in the ATC data.
5. The 2nd plane make a U-turn using MH370 transponder signal as a decoy confusing the ATCs in that area.
6. Why late at night?
7. Mid-night after 12 not many ATC controllers on duty, relax one corner.
8. All passengers are sleeping and pitched dark outside and not suspecting anything to happen. When they wake up in the morning, Mr. Kim will be greeting them welcome to North Korea, red carpet.
Why in Asia?
9. Where else? fuel is the reason.
9. Which country has mid-night flight out of the country, Malaysia and Singapore?
10. N Korea is Asian country, and choice of stealing a plane must be in Asian route, and the plane is heading north straight path, and after mid-night flight and fuel reasoning.
11. Not likely steal plane across the Atlantic Ocean from US, fuel problem.
12. Not likely steal a plane which is heading South, East or West, increasing suspicion with plane make U-turn?
13. Not from China either, Chinks too good for them and offend Chinks sure kenna jia jia lat afterwards.
14. Malaysia offers them best option to take this chance.
If you are referring to the 777-200's tank capacity then the answer is yes, but it would depend on the amout of fuel that the pilot uplifted. In this case the pilot would have taken enough fuel for the trip to Beijing so purely from the fuel point of view, if he wanted to change course to fly to Pyongyang then it would not be a problem fuelwise.