CZ flights can change seasonably, I suggest using Ctrip to check with CZ's major hubs around (CKG, CAN, DLC, PEK, SHA/PVG, SHE, CTU) or use the ST tool:

http://www.skyteamtimetable.com//update.html?20110219

As for seasons, Jiuzhaigou has a different experience for each season, so you can't go wrong going in different seasons. All are beautiful from what I hear.

I never stayed at the Sheraton, but keep in mind that the Sheraton is VERY close right outside the park while the IC will require about a 30 min drive from the IC to the park entrance. Not bad, but figure it in with your trip. Again, I suggest finding a driver the whole time you are here.

As for timing, 2 days is doable if you want to see the majority of the park and move fast, 3 days is good. If you want 4 days, I suggest seeing some of the other sites in the area. Again, depends if you want to do the hiking at all, that of course will take longer but is a better experience IMO.