On #4, that's one thing I noticed too. No mayday. No emergency declared. No contact. The flight just failed to show up.

I can think of three cases where this could be. The first is where the pilots tried to communicate that something was wrong and were unsuccessful, but I would expect that someone might have heard something.

But the other two are more catastrophic. The plane could have disintegrated in the air, giving no time for communications. Midflight breakups are uncommon but they do happen.

Finally it is possible that the pilots for some reason didn't know they were descending and could have crashed into the ocean without ever knowing they weren't at altitude. This apparently happened with AF447 to some extent. It has happened before as well (static ports get clogged, pilots get disoriented at night, etc). In this case the first sign that anything was wrong could have been impact.