You're going really out of the way to squeeze in Takayama on the way from Hiroshima to Nagano. And then you come nearly a full circle back to Shirakawa at the end, which is basically a stone's throw from Takayama. Maybe go straight to Yudanaka from Hiroshima (even though it'll be a long train ride). And then what do you think about using Toyama as the base at the end to make day trips to Kanazawa, Takayama and Shirakawa? Or just stay in Kanazawa and make day trips to Takayama and Shirakawa from there.

I've driven between Matsumoto and Takayama during the winter a couple of times (actually someone else was driving but anyways...). I recall that the roads were clear both times. However, that may not always be the case. It can snow quite a bit in that area. The bus ride is nice, particularly between Hirayu and Shinshimashima, but I've only taken the bus on that route during the summer.