Does safety belts trap passengers in burning or submerged cars?

Answer 1

It can happen that way, if the belted person for some reason is unable to operate the release mechanism.

However, the risk of getting ejected from the car(and badly injured) or getting thrown into the dash/windscreen (and getting badly injured by that) is far, far greater than the risk of getting trapped by the belt.

So if you choose to ride unbelted for fear of getting trapped, all you're doing is trading a really small risk for a much bigger risk, which isn't a sensible choice.
It is possible to have your seat belt jamb but it is very rare and the odds of dying from being ejected from the car are way higher than being drowned or burned. Some hard headed people still don't believe that but ask yourself this question: If you were going to get in a jet with the Blue Angels would you buckle up? Sure, you would be crazy not to. Guess what? There is nothing going on in a Blue Angels jet that is any more violent than a car crash. If you are concerned about being trapped by your seatbelt, go to your nearest autoparts store and buy a seat belt cutter to keep in your console or glove box. Even a box cutter will work.