Most studies showing a decrease in head injuries due to wearing a bicycle helmet are, because of insufficient statistical methodology. So a meta-study is most likely bullshit, too.
To your credit, at least you didn't cite Rivara/Thompson/Thompson directly - but their studies are referenced in the paper you mentioned.
So, the situation is this: Yes, a helmet can reduce the severity of head injuries from a cycling accident. It's no guarantee, though, and depending on the situation a helmet can also increase the risk of serious facial injuries or worse, a broken neck.
BUT: The danger of being involved in an accident is not high enough to warrant the loss of comfort induced by wearing one (disregarding the cost, which isn't high enough to matter). Just have a look at countries where many people ride bikes (NL, DK - hey, you live there): Almost nobody wears a helmet. That should tell you enough, really.
If you are afraid of having an accident, do something about that by learning how to ride a bike in traffic properly. Yes, I'm assuming you can't, because most people can't, even in a bicycle-heavy country like Germany. Learn what the risks are and how to avoid them. Encourage others to do the same. That's a far better investment than a helmet. Of course, you can wear one in addition to that. But it's not as important as you think.
I am somewhat emotional on this topic. I view riding a bicycle first and foremost as a mode of transport on short distances, and as a good way to avoid too much car traffic in town. Riding a bike is fun, good for your health and avoids pollution.
Essential to that is cycling being easy and straightforward. Anything making it more complicated is bad. Having to deal with a helmet (probable/possible negative effects: having to take it with you at your destination, ruining your precious hairstyle, looking ridiculous) is a complication.
Even more important in the grand scheme of things: The more people choose cycling over driving a car, the safer it is to ride a bike ("safety in numbers"). Seeing people cycling only with helmets makes people think of cycling as an unsafe mode of transport and thus discourages non-cyclists from becoming cyclists. Making wearing helmets a social convention is therefore a bad move for cyclists. Australia went one step further and implemented a mandatory helmet law: The result was: 1) less people rode bikes, 2) the risk of an accident for the individual biker increased (because car drivers were becoming less used to bikers - the opposite of "safety in numbers"). The current state of science on this topic is that promoting helmets has a negative effect on public health - the positive effects of reduced injuries are outweighed by the negative health effect of people not cycling at all.
Conclusion: If you want to ride a bike, do so, but do it safely. If you want to wear a helmet, by all means, do so. But don't tell others to wear one as well, because chances are they use a car instead (and perhaps crash into you).