it is not unusual for terrorists to NOT claim an attack for a while afterwards; as a matter of fact, Bin Ladin did NOT claim responsibility for 9/11 until sometime in 2004 (iirc), and denied it many times previously. (However, it was well known that OBL was, in fact, responsible, due to many, many links connecting him/AQ with the plotting, the 19 hijackers, and many other aspects of things).
And, in more recent attacks, the Benghazi attack STILL has not been 'claimed'.
So no, a claim is not always quick in forthcoming. I don't know anything about Chinese terrorists, so can't find a track record for them claiming/not claiming their attacks, but even though we think of terrorists claiming responsibility for attacks right away, that's not always the case.
I still go with my theory of some sort of baggage compartment explosion, set to detonate at 35000 feet (cruising altitude). This is similar to the Locherbie, Scotland plane explosion, except that it was over water and not over a town. If you consider how widespread the debris field was for Locherbie, and add to it ocean currents and items sinking due to both gravity, saturation and the force of impact, then you may see why it would take a long time to figure out where the plane came down.
And MsF., it would seem to me that oil, being more buoyant than water, would rest on the top of the ocean; ocean surface winds as well as currents would definitely 'spread' out the oil slick, and could easily be the reason the slicks are called 6-10 miles long.
Best-
Herding Cats