Fenghuang to Hong Kong

You may book the bus ticket from FH to Changsha, so on the Sep. 12 morning, you can take the bus to reach Changsha first. In Changsha, you can try to book the flight tickets to HK directly. Or you can take the flights to reach Guangzhou and then to HK.
Hope you can find the flights between Changsha and Guangzhou or Changsha and HK.

Hi Jenny, thank you for the answer. Is Changsha better/faster to get to than going to Zhangjiajie? Have you taken the bus from Fenghuang to Changsha before? I hope I can get a seat, I know how hazardous getting a reservation is!

The distance between FH and Changsha is far than that between FH and Tongren, but I think it is much easy to fly from Changsha to Guangzhou.
Yes, I have taken the bus before and it is suggested to book the ticket the ticket one day before.

Ok that sounds good. I see that you can also book the ticket for the bus online through this website. I think I might do that. thank you!

Frankly there isn't enough information to sensibly answer the question. When does your flight (I presume back to the west) leave HK? My gut feeling from reading your post is that you're cutting this whole thing awfully close. The most reliable transportation method in China, IME, is train. If there's a train timetable that gets you where you're going (search the internet) then go that way. If not, well, I feel like you're playing with fire here if you MUST make it to HK at a specific time. To be honest, if you are not able to find a train schedule that meets your needs, I feel that your schedule is too aggressive and you might need to drop FH from your trip.

In other words... go by train if it is important you get to a specific destination at a specific time.