First trip to China (16 days): Beijing, Guilin/Yangshuo, Zhangjiajie, Harbin, Shanghai in Dec/Jan, sanity check from people who've actually done this?

My fiancé and I are planning our first trip to China, 26 Dec to 11 Jan. We're a young couple who loves hiking, adventure, and nature, and we specifically want to see snow (coming from Sydney, Australia). Here's what we've got so far:

• Beijing (3 nights): Great Wall (Mutianyu), Forbidden City, Temple of Heaven, hutongs
• Guilin/Yangshuo (3 nights): Li River, Yangshuo cycling/hiking, Moon Hill, spending NYE on the riverfront
• Zhangjiajie (3 nights): Avatar Mountains, Tianmen Mountain, Glass Bridge, hiking
• Harbin (3 nights): timed to land right around the Ice and Snow Festival opening ceremony (Jan 5)
• Shanghai (2 nights): Bund, Yu Garden, French Concession, easy finish before flying home

Questions for people who've actually done winter China travel:

1. Is 3 nights enough in Zhangjiajie in winter, or does weather (fog, closures on the glass bridge/cable cars) mean we should budget more buffer?
2. Anyone done the Guilin to Zhangjiajie train? Worth it, or should we just eat a flight connection somewhere instead?
3. Is Harbin in early January (around the opening ceremony) worth the hype, or is it overcrowded/overrated compared to going a bit later?

Would love real experiences or any feedback!