Interesting about taxi drivers. How awful that people would use a seatbelt to try and strangle somebody.
Did you know US airbags are bigger than European ones because they are designed to work even if the driver is not wearing a seatbelt? European ones are smaller, but if you're not wearing a seatbelt, you probably won't be in the correct position for the airbag to work.
I think the thinking about it being selfish even when alone is that you could injure people outside the car if you are ejected from it, plus it would be more traumatic for witnesses/emergency respondents if your body is more mangled. But not as big a problem as not wearing one in the back.
Under 12s in the back only applies when you're talking about children travelling without car seats unless they are tall enough not to need one. They can be in the front with a car seat (forward facing only unless airbag is disabled). In the back, a child (over 3) can be without car seat if you can't fit three across, it's a classic car without rear seatbelts, it's a taxi and you don't have a car seat with you (also under 3s), or it's a short, necessary and unexpected journey (and you don't have a car seat with you). The RoSPA site stresses that these exceptions are made for practicality and not with safety in mind, it's not magically safe to do these things, so you should try to use car seats if you can.
Valhalla that's really sad :( Seatbelts unfortunately aren't really designed for women but for the average man, which means sometimes shorter women (teenage girls?) or those with big boobs can find the belt routes in an unsafe way. There are products you can get which help, similar to booster seats, but they aren't very well known and I'm not sure they are properly regulated and crash tested.