Somebody else commented that the battery life isn't exactly 30 days. But a battery whose expiry date hasn't lapsed must not in the testing phase fail before 30 days. Good engineers always add a factor of safety margin to pass the tests so it would not be surprising that the pings continue for a few weeks or a month.
Both cockpit voice and flight data recorders with pingers are located in the tail which in the event of a crash are not subject to the higher g forces of the front of the plane. There is a maintenance data recorder in the cockpit area but it is not designed to be survivable nor does it have a locator pinger.
It is totally nonsensical that a non-Chinese airliner would land in Hainan (a Chinese island province) to off-load a secret cargo. It would not arouse any suspicion to offload this cargo at Beijing and put it on the next scheduled domestic flight to Hainan.
Why would you need an airliner the size of a Boeing 777 to carry a nuclear weapon? Nuclear weapons, no matter how crude, simply aren't very large objects.
Then I would say a Boeing 777 is a good disguise for a few nuke suitcases.
There is talk about a structural failure. Composite failure?
Then why did the first ones require a B-29? Fat Man weighed 10,500 pounds.