I would suggest that the odds of getting tickets on the Sunrise Seto at that time of year are very, very slim and you will likely also have issues on the Tokyo-Nagano which you plan to do on the morning of December 30th. The Tokyo-Nagano trains will likely all be sold out by for that date , so you better book that as soon as you land. I would actually forget about the Sunrise Seto and just figure another way from Hiroshima to your final destination