flight to Jiuzhaigou

Hi, is it true that in December only flights to Jiuzhaigou go from Chengdu, not from Xian, Shanghai, Chongqing, ... ?
Is it recommended to take the flight to Jiuzhaigou from Chengdu and then travel back by bus? Or is going (as well as returning) by bus not so much more time?

Will all the parks / attractions north of chengdu (Jiuzhaigou - Huanglong - Mount Qingcheng - any other recommendations?) be open in late December? And will all busses drive (where can I find a time table?)

Thanks for all help, you guys are doing an amazing job!!