"And OK, their flights might have been booked well in advance, but the timelines for planning an operation to take over a plane ..."
I'm pretty sure it would be easy to target a certain flight on a certain day for a certain reason. You only have to have soemone who knows how to watch the GDS.
"Shouldn't that be 'US citizens who work for a large company some of whose products have military applications"?"
I'm not sure that's really so different, is it?
"And it's also a whole different league of operational planning (and keeping the plan covert) than a massed knife attack"
Completely agree. Like I said, gut instinct, timing conicdence and all that.
"Which is why I tend to keep coming back to Indonesia,"
Exactly, except that there is a hugely powerful radar system that is know to monitor Indonesia's airspace (that doesn't belong to Indonesia.)