I've got great faith in planes as a whole, but these sudden vertical accelerations (wind pockets etc) is one area where they don't seem to have much of a plan aside from hoping you're wearing a seat-belt at that particular point in time.
Maybe they should add something crumple-zone like in the panel above the passengers heads. Or put something air-bag like there.
I don't think they can add much crumple zone up there - that area has to be strong enough to hold people's luggage.
(On another note, I'm a very nervous flyer, and I put my seatbelt on as soon as I sit down, and don't take it off unless I'm using the restroom, or until we're at the gate. Hurray, paranoia!)
Effectively never. The fuel is kerosene which has a high flash point, and the plane is grounded to the refuelling apparatus to prevent any static sparks.
Maybe they should add something crumple-zone like in the panel above the passengers heads. Or put something air-bag like there.