The Kunming-Beijing part of the trip, including time spent at Zhangjiajie, is going to hinge on getting that Zhangjiajie-Beijing train ticket you want--or finding an alternative strategy. A look on the official Railways website (Chinese only), reveals that it appears tickets on these two trains K268 originating Huaihua and K968 originating ZJJ have advance sales windows of 5 days only, not 10 days. I entered values for 5 days from today, and there are some sleepers left (esp hard sleeper on K968 evening train), but I'm sure those will get soaked up as the actual departure date approaches. And you will be traveling after a holiday (January 1-3 is an official holiday in China). This frankly, does not bode well for you to successfully purchase a ticket by yourself on either of these trains for either Day 17 or 18 of your trip, since in Kunming you'll be too early for sales and once arriving in ZJJ, likely to be sold out. You could try buying at Huaihua rail station when you arrive from Kunming, before heading to bus station. But...no guarantees and it would make me a little anxious unless I had backup cash to throw money at the problem at book a last-minute flight to get me to Beijing.

I suggest you start thinking about alternative strategies now:
1) Locate the hotel or guesthouse you want to stay at in ZJJ, and have them get a ticket if they agree (they will surcharge). There's always some angst about this, as to whether when you arrive, they will have actually done it successfully.
2) Consider flying from ZJJ even though expensive (over RMB 1000). At least you could do this in advance and KNOW you will be getting back to Beijing in time. From ZJJ, the CZ 17:15 on January 3 is probably the way to go, which would give you 5 extra hours in ZJJ area compared to your first-choice train. You'll likely get better pricing if you wait to buy until just before you leave for your trip, or just after you arrive in China, it's a bit early now. Flying afternoon of January 3 for BJ arrival that evening will give you an extra day (the 4th) as backup in case of bad weather or flight cancellations. Always adviseable if you have an international flight at 2 am January 5 that you must make.
3) Consider getting to a better transportation location, such as Changsha. Buses now do the ZJJ (city)-Changsha route in about 4 hours, and leave frequently throughout the day. You could leave ZJJ by bus on the 17th around the same time you had planned for the midday train. Once in Changsha and allowing for time to get to either rail station or airport, you have some evening options for getting to Beijing the next day (train) or that evening (plane). Lots of good trains out of Changsha--the Z18 and T2 originate there, and about 5 more T trains passing through. These trains should definitely be bookable 10 days in advance, and possibly at the Kunming station. If you want to do plane out of Changsha, the flights to Beijing are often fairly heavily discounted, and if purchased about 2-3 weeks in advance of flying, can be had all-in for 700-800 RMB.

I would have my exit strategy from ZJJ (or Changsha) in place well before arriving in ZJJ, so that you can then backwards-figure the time you have between getting off at Huaihua station, Fenghuang, and ZJJ. You may have to make some real-time field adjustments based on the ticketing strategy and situation.

And yes, I believe there is a separate entrance fee for Tianmenshan. And that the Wulingyuan park is the one that the ticket is good for up to 2 days. But double-check on that, as my info may be a bit dated with the new practices. Like that fingerprint machine thing tauphi mentions above--friends of mine went to ZJJ earlier in 2011 and that didn't exist then.