Today I got in contact with my predecessor. In addition to answering my many questions, he sent me some pictures!
Now that's some real snow!
Despite the many sources I've read saying that Japanese buildings have no central heating, he told me the ones here do. Which makes sense, looking at these photos. He said that the temperatures sits around -15°C at the worst times, though the lowest he experienced was -25°C.
Also, apparently there's a ski slope right behind the house. Maybe I should learn to ski...