Some Questions about Kyushu on December

I am flying to Fukuoka next December, being my fourth time in Japan. I have some questions, some of them a bit unusual.

*I wil use the dedicated Kyushu JR Pass, but would like to see the special Evangelion Shinkansen 500 from JR West at Hakata station. I assume that my pass will not allow me to ride that train as the Sanyo Shinkansen is not included, but I think I could reach its platform in order to take some pics. Do you know if JR West's Sanyo Shinkansen has dedicated gates, or they are using the general ones shared with Kyushu Shinkansen?

*Concerning about autumn leaves, I assume that it will be late for most cases, but in Japan Guide it is stated that in some places you are still on time at the beginning of December. Considering Kyushu's milder weather and that my flight lands in Fukuoka on the third, maybe I will be able to see any momiji/koyo. I found some information about Dazaifu but was not able to find any other places, could you recommend me any around the island (if possible, easily reachable by train).

*I plan to use as many JR Kyushu special trains (Yufuin, Aso Boy, Take the A-Train...) as possible, in fact, my itinerary is being conceived around its operation days, but I'm afraid of not being able to ride if they are fully booked before my arrival. Do you know if I have a chance to get any seat between 3 - 7 days before train departure? I think it is not high season (nor holidays) for the first half of the month.

*About Aso, concerning the actual situation of the volcano (and without any hopes that will be open the next months), do you think it is a good idea to stop over there, or just see the region from the train when going to Kumamoto from Beppu? I am no interested on horses.

Thanks a lot in advance!

Shinkansen part of the station has all shinkansens. There are also shinkansens that go from Hiroshima all the way to Kagoshima via Fukuoka's Hakata station. So yes.

Kuju mountains. Kirishima. Takachiho has some, but the amount is too little for the journey there.

You probably are very likely able to book tickets for these trains.

There are a number of spots in Aso. But they are better reached by car. By train, it is just to see the nice scenery. Rivers and mountains and your autumn colours.

Thanks a lot Joshua. Takachiho was already on my schedule, so I'm glad to hear that.

If you are arriving early in the first week of December, then you still have a shot at seeing some autumn colors. Depends on the area though. One of the better (yet lesser known) areas is Akizuki, referred to some as a "Little Kyoto".
http://www.youtube.com/watch?v=S0-YxwfD340
It's a small town area but has a lot of people that go see the beautiful autumns there. Another place that not even many of the locals know about is the Kiyomizu-dera Temple in Miyama, Fukuoka (not to be confused with the more famous one in Kyoto).
https://www.youtube.com/watch?v=jKAMjJ4VrTo

Needless to say, you cannot take a train to Takachiho. You can catch a bus from Kumamoto if you want to use your Kyushu Pass part way, or get on the bus in Fukuoka - there is not much time difference overall either way.
Sadly your chances of seeing Aso close up are very low. Before it blew its top there was a 1 km exclusion zone for over 6 mos, so even if the place went completely quiet today, your odds of seeing the crater face to face are slim to none. You could still go up to Kusasenri if you think it's worth it. If you'd like to get a glimpse of what you'll miss though, have a look at
https://www.youtube.com/watch?v=Tyh_nKrv5tY

You are correct though regarding your time in Japan will not be during a peak period season. If you want to reserve a seat on Aso Boy for example, it doesn't have to be done on the day of departure. Just go to the JR office and they'll set it up for you.

If you want to enjoy Japanese nature, why not spend a day and night in Kurokawa? It is one of the best hot spring areas in the country.
There is one other famous temple for autumn colors, between Karatsu and Fukuoka City, in Itoshima. The Raizan Sennyoji Temple is a gem of a place and unknown to all but the most traveled and informed. Mid-November is usually best but you could still get lucky. You'd need a 20 minute taxi ride from Chikuzen-Maebaru Stn. especially before 10 AM when the roads get jammed. The Kanzaki Garden in Saga would be great as well, but you just won't be there when it's open to the public. Since you'll be in Fukuoka to see the shinkansen be sure to see the Nanzoin Temple, just 20 minutes by train from Hakata Stn. Absolutely beautiful.
https://www.youtube.com/watch?v=lT4VQFmHFyM
There is a lot more you can enjoy as well - see
http://www.thejapanfaq.com/fukpic.html

Thanks a lot Ken for your extensive answer.

Yes, I assumed there is no train to Takachiho so I planned to go by bus from Kumamoto, the Aso region or even from Fukuoka if my railpass is not active yet. I am impressed with your recommendations around Fukuoka (and the sites described on the website you linked), definitively I will spend more time over there. About Kurokawa onsen, it was on my top list but at this moment I was not able to find any affordable accomodation over there.

Thanks again.