Simplest theory is the pilot or co pilot maybe even together due to whatever was in their heads took the plane out. Everything else listed gets too complex and crazy. It starts flying, loses contact, does some odd turns away from scheduled flight path, starts to avoid detection by flying low and presumably crashes somewhere into the Indian Ocean. The fact that it wasn’t found as such just proves its a very big ocean area to search.
Just last night, I was asking a 777 pilot what the crews he flew around the world with thought had happened to MH370. He said, the general theory is that it just blew up.(period) A chain of mechanical problems, or even a bomb.
The suicidal crewmember theory was an attempt to deflect blame away from the equipment. Airplane manufactures can’t afford to have fingers pointing at the equipment (ala 737 max).