"Missing" is an euphemism for "very likely crashed" here. They won't say crashed just in case it is later found with people alive, but not communicating and gone from radar over the sea usually ends only one way.

It could be hijacked, landed, and held hostage, but it's incredibly difficult to hide a 777 anywhere in the world where it can safely land. It's possible it was hijacked with intention of being held hostage but crashed, hence no claims of responsibility.

Not likely. It was at cruising altitude and then vanished off the radar screen after sudden maneuvering. If it had diverted it would have continued to be tracked on radar. Even if they had shut off transponders and such, primary surveillance radar would have continued to see them. It almost certainly experienced a sudden catastrophic failure in flight (and I would bet due to a bomb).