"but what we do know is that it's extremely rare for a cyclist to suffer a serious injury or death from a simple fall"
Based on what evidence? I would argue that most accidents involving any kind of injury to a cyclist aren't necessarily logged and reported. A simple fall could happen at 5mph or 30mph. I dont remember the speed I was doing when the accident happened, but it was far more than walking pace and from the dent in the helmet, I can assure you my head did not land on a flat surface.
As for helmet foam compressing easily, I don't about yours, but mine isn't made from jello. Helmets are designed to compress so that your head doesn't.