They probably crashed. Most likely, the plane is on the bottom of the sea. They just haven’t found it yet, but sooner or later, they probably will.
Granted, the sea doesn’t always yield her secrets. So we may well never know for sure what happened.
Quite possible yes.
I don’t know much about planes but #21’s first scenario seems more likely than that second scenario.
Since jumbo jets can be used as weapons the plane has to be tracked down even if finding it is unlikely. Terrorism cannot be counted out.