Day 16 isn't going to work. There's no way you are going to get from ZJJ to Changsha West Bus Station to Changsha Rail Station in time to buy a ticket and make that 11:30 D train. You'd have to leave ZJJ by about 6 am and there won't be public bus going out that early. Nor should you want to be on those roads in the dark--too dangerous. My suggestion would be to try to grab something like an 8 am bus to Changsha, arriving around lunchtime, then change buses (you may or may not have to change to the East Bus Station) for Nanchang and forget about the train. You'd still get to Nanchang by sundown on day 16. Unfortunately, there's no Changsha-Nanchang train in the early afternoon and the next train is way later.

I personally would choose to spend an afternoon/evening in Fenghuang rather than 3 nights in Nanchang if it's just a casual friend you are visiting. Only a very dear friend or family member would have me spending 3 nights in Nanchang.