To be fair, a lot depends on how you sightsee as well. Are you spending 2 hours at a temple? 1? 30 minutes? or 3 hours? There is no correct answer and different people will spend different amounts.
Personally, we spent less time at say Kinkakuji then we spent at Byoudoin. These all can make a difference on whether your plan might work or not though.
So for example you have Himeji and Kobe the same day. Well, whether that works depends on what you plan to do in both and when you plan to try to arrive in your first destination. I personally tend to be the sort of person who prefers to be up and out early in the morning and be in bed early. But I don't know how you like to travel and that can play a huge role. I could visit Himeji Castle and possibly Koko-en and Kobe in one day, but I'm also not overly into Kobe and just tend to go there for shopping. I could not do the castle, the garden and Mt. Shosha and Kobe, unless I was just going to Kobe for dinner.
I think Hiroshima and Miyajima as 1 day returning to Osaka/Kyoto is 1 very long and exhausting day. We took a 7:40 Hikari from Kyoto to Hirohima, caught a train to Miyajimaguchi within minutes of getting off the shinkansen and by the time we got back to Hiroshima Station it was 3ish. But again, it depends on what you count as visiting both and how you spend your time.
If you have express passes for USJ, yes it makes a huge difference, but they can sell out during busier times unless you can buy them in advance. When I was there on a "quiet" day, outside the kiddie rides most rides still had 1 hour+ lines by lunch time. We rode Jaws first thing in the morning and had no line, but by noon or 1 the line was over 1 hour. I personally didn't think the wait to get in was that bad. I want to say we were out there by 8:30 for a 9:00 opening?
I personally find DisneySea gets even more crowded than USJ and lines very quickly get very long.
I also agree with previous poster in terms of Takayama it partially depends on what you want to do there how much time you need there. It also depends on when you plan to arrive.
In terms of the Ghilbli musem... you will be paying a HUGE premium for the "tour" but if going is a super high priority for you and you don't mind paying much more than the cost of doing it on your own, you can book a tour via Japanican for December still.
http://www.japanican.com/en/tour/detail/F601/December 8 is still available. But it depends on how important this is to you. I am 99% sure Japanican is a subsidiary of JTB, so they are reliable and you will get your tickets.