life jackets
Read the NTSB report into the Hudson ditching with regard to life jackets.
They note the dificulty in extracting life jackets from their retainers and the understandable reaction of the passangers not to continue trying but to get out.
Now imagine doing this on your own initiative probably in the dark whilst in a turbulant descent. Further the instruction on use states: do not inflate until outside the aircraft.
With regard to bodies seperating from seats whilst strapped in there are plenty of crashes on land where this has ocured - why so unlikly on water?
Jumpers from tall buildings often bounce
Wtech the captain would not have left the cockpit under these circumstances and if he was in the rest area he would have been trying to get back to the cockpit if possible.
With regard to the high number of cabin crew - observe what they do after a service. Usually congregate in their galleys and chat. This group could have been in one area that allowed their bodies to escape the fusalage.