Isn't the overt response of the US and other possible target nations (India, Pakistan, Saudi Arabia, Afghanistan, China etc) extremely muted to the possibility, no matter how remote, that terrorists somewhere just might have a 777 in their posession?
Not really, or?
From all the countries in your list, only China is the one where the aircraft could have actually gone to. What would you expect China to do? Admit that they saw no unidentified AC on their radar? Or say that they saw one, but didn't care? (How long are military radar tracks stored anyway? Days? weeks?)
Just imagine a terror attempt gone bad. Instead of putting a plane down with 280 dead bodies (of whom 150 are chinese) at the end of the runway in Urumqi, the terrorist didn't make it and crashed somewhere in the Xinjiang. Would China ever announce that they found the plane if the would find it there?
And use the 777 later? what do you do with the dead bodies? If not removed, you don't want to go near that plane after the 8 missing days now.