Driving on an iced lake or any other condition where your car is more likely to become submerged than you are to run into something, being unable to exit the vehicle quickly can be fatal.

The typical example is an underage child in an adult seatbelt. Because of the difference in dimensions, a crash causes the seatbelt to impact the neck.

I'm not sure that would be classed as "used properly", though.

Is it a legal requirement for underage children to use an adult seatbelt? I know here in Sweden for example it is illegal to have child, under a certain height, in a car without proper child seatbelt/car seat. Form a legal point of view no seatbelt and adult seatbelt are more or less equivalent.

Here (Australia) children have to use a correct child restraint and/or seat up to the age of 7.

I believe the legal point is the same as you've stated - the law is "not correctly restrained" or something.