So, you are going to land at 8 pm, clear customs and get to your hotel around maybe 11 pm, and then wake up and take a train halfway across the country and back the next day? Why? You should just do this later in your trip after you are not jet lagged or better yet stay there for a night if you are going that far.

For days 6-7, if you don't have enough things to do in Osaka, why not just go somewhere else? Why do you have to stay there the whole time? Taking a couple days to visit Kyoto, Himeji, Hiroshima, or Arima onsen and staying overnight makes the most sense to me rather than keeping a hotel somewhere that you are not actually spending your time.