I agree with your first two comments. As to the third, appointing a "disaster successor" is rarely done particularly when administrations change for one very practical reason...the incoming POTUS intends to appoint his own cabinet. In this particular case, Obama asked Gates to stay on for at least a couple of years so Gates is the only member of Obama's cabinet who is in place and confirmed.
While I hope we don't need a disaster successor, I must point out that a terrorist attack would have to wipe out Obama, Biden, Pelosi, Byrd (Gee, Biden is Byrd Brain and Byrd is just Bobby Big Byrd Grand Kleegle or whatever of the United States of AmeriKKKa as Jeremiah Wright would note.) for Gates to succeed to the presidency.
Imagine the pressure of knowing that if things go to hell in a handbasket, you´re up!