"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).
Or not a bomb, just some maintenance problem. No group claims responsibility. The point of bombing seems to be to strike fear and send a message. The passport thing is curious but stolen/fake passports can be of use for other things that are nothing to do with terrorism.
Indeed. My money is on "bomb" just because it's hard to come up with a maintenance problem that causes such a rapid disaster at altitude. It would have to be something that caused a massive explosion like TWA 800. Possible, but unlikely. The lack of a claim makes a bombing less likely too, but IMO less so than the improbability of a mechanical problem.