Getting there
There are no direct flights between China and Mexico. Chinese visitors have to fly to Tokyo and catch AeroMexico's flight from Tokyo to Mexico, en route Tijuana. According to the Mexico Tourism Board, however, AeroMexico will launch direct flights between Shanghai and Mexico City next March. There are regular flights from Mexico City to both Merida and Cancun.
Uxmal is 80 km (one and a half hour's bus drive) from Merida. The site at Uxmal is open daily from 8 am to 5 pm. If you really want to watch the sunset on the Pyramid of the Magician, you can walk into a small lane and walk out after closing time.
You're better off staying the night at the town near Chichen Itza and do your exploration of the site either early in the morning or late in the afternoon. Leave yourself more time to swim in Cenote Ikil, which is only 15-minute drive from Chichen Itza.