The pilot had decades of experience and a flight simulator in his home where he could practice such things. If he was involved, he probably could have pulled it off.

...how could the potential hijackers know that the Malaysian military wasn't properly monitoring the radar?...

We don't know what the contingency plan was, because fighters weren't dispatched. Maybe pulling off this plan was a million to one shot, and they just got lucky.

IMO, the only reason that one would try for stealth in this situation is if their intention was to land the plane somewhere. If their intention was to crash, they didn't need stealth or 7 hours of flying time. They could have pushed forward on the stick for a few seconds and dove the plane into the ocean.