Quite exciting, Piglet. The post-dated invitation letter is a new one for me. Obviously the school does not want you to arrive in China before then, and the consulate may honor that.
As far as the air ticket is concerned, a person can buy one at any time, regardless of visa status. Normally, waiting until the last minute to purchase would not not be a wise choice for someone price-sensitive, such as myself. In addition, an open return ticket costs more. In my opinion, it makes sense to choose the return date now based on current information, knowing a change fee might be necessary down the road. Airline ticket prices are a real crap-shoot.
Good luck!