Certainly no expert on the topic, but as regarding flying passengers at night, I guess my first question would have to be... why? What competitive advantage is to be gained? I guess in the N. Sea it may be because the daytime is so very short in the winter.
There are a lot of factors which probably would weigh the accident statistics, whatever those happen to be. A couple off the top of my head include:
1) Proficiency. Since, as you point out, we don't fly that much at night, whether we like it or not, we're not that good at it, at least not when compared to day abilities. When instructing regularly at night back in a former life and flying a substantial percentage of my hours after dark, my proficiency level was quite high, but I was never under the illusion that there were not things out there ready to bite if it all went pear-shaped. This argument has two sides, really. Since, as you point out, we don't fly that much at night, we're probably not that proficient. However, if we did it more, we'd probably become more so. Current percentages would probably skew the statistics toward a higher accident rate and higher ones would probably move the rate lower, but again the operative question is why do you want to do it in the first place?
2) Fatigue, Circadian rhythms, and Reverse Cycle Operations. See FedEx. They have an entirely different crewing model than an operator who mostly flies during the day. This issue almost nipped FedEx in the bud before it ever started... nobody would insure them with a standard airline crewing model.
3) Emergencies. While the crew may be capable of ditching the machine day or night, the elephant in the room is what about the pax? The after-landing part of the ditching is probably going to be messy.
4) For what little it may be worth, the U.S. Navy prohibits overwater passenger transport flights in helicopters at night.