Perhaps someone can clear this up for me, but WHY in the world is seatbelt jamming such a common problem?
You'd think they could design a buckle that would still function after an accident.
They do not "jam". There used to be a serious problem with seat belts coming undone in a crash. After a lot of research was done (mostly in the U.K. by the Department of Transport) the modern seat belt was designed. It is designed in such fashion as to ensure that if the lock fails the parts jam closed.
It should be appreciated that in 99% of all crashes the safest place for the driver or passengers is in their seats.
Situations like the present tragedy are the exception, not the rule.