I haven't taken that flight for a bit over a year, but here are my experiences:
- You'll get a small appetizer (no choice) and a choice of 3 hot meals. I think typically one western, one asian (usually chinese) and one vegetarian. The quality is average for UA, abominable for Asian carrier standards.
- I used the Airport Express checkin at kowloon station, so I'm sure Central will be fine.
- No idea, but there would be no UA specific restriction I'm sure. Check out the HKIA website.