Had I gone fully over the door head first, I suspect a helmet would have been a welcome companion for both the impact and the road rash. In all those above cases I was wearing one precisely because of that suspicion.

As a lifelong cyclist, I can assure you there is no way to mitigate hazards enough on a bike, even in as cyclist-friendly place as Amsterdam, to justify not wearing a helmet. Even something as simple as an awkward fall can result in a head bouncing off the pavement.