Shanghai South Bus Station has direct shuttle bus to Tangkou, 6 hours or so. I suggest you take this bus. There are 4 buses a day to Tangkou every day, two in the morning. Buy ticket one day in advance, adjusted to the timetable of that day. Get to the hotel in Huangshan and then climb the mountain next day. Stay another night in Huangshan and take the morning bus (7:20, 9:40)to Yixian. These bus will pass Xidi or Hongcun. After visiting Xidi and Hongcun, you can get back to Yixian. Then back to Tunxi. The latest bus back to Shanghai from Tunxi is 15:50. It takes 40 minutes from Tangkou to Xidi, 1 hour from Yixian to Tunxi. If you want to take taxi, it costs 100RMB from Tangkou to Hongcun/Xidi. You can take taxi to Xidi and Hongcun after arriving at Yixian.