I want to go Zhangjiajie to Fenghuang, then go to Guilin.

1) It’s about 35 miles and duration is about 60 minutes by bus. BTW, you need to go to the long-distance bus station to the west of Jishou train station to take the bus.
2) Yes, you are right about the bus and its schedule. Generally speaking, there are enough tickets left and there is no need to buy one in advance.
3) There is no train from both Zhangjiajie and Jishou to Guilin. The closest departure station should be Changsha. In this case, the recommended route should be: town to Jishou by bus, then to changsha by train, then another train to guilin.