Transportation to mutianyu greatwall and any public transport to 798 art district?

Yes, 16:00 is the last bus. 867 runs between Dongzhimen and Hongluo Temple, only to MUtianyu directly at 7am, 8:30am and back at 14:00 and 16:00. If you miss the time, you should get to Huairou first and take other 867 buses back to Dongzhimen, or bus 916. The faster way back to the city is to take 867 or others back to Huairou (or taxi) and then change to 916 which is an express bus going trough expressway. So, it will be faster. Get to 798 art district from Dongzhimen? If you go to that place just after Mutianyu, you can just get off 867 at Dashanqiao Dong. Then walk a few minutes (about 900 meters) to the art district. Take 916 express, you will get to Dongzhimen directly and then take bus 401, 955, 909, to Dashanxi Lukou Nan (大山子路口南)station.