Ngaire, we won't be taking you up on China trip but I have some recommendations:

Include Tibet; absolutely worth time, expense, and lack of oxygen.

Include Chengdu and a trip through countryside to Leshan.

Yangzi Explorer was a fantastic ship with just over 100 pax but there might be even nicer ones now. The four night up river has lower demand than three night down river so it can be less crowded.

Take control over your meals. When you are on expensive (comparatively) tour, they tend to want to order the expensive lunches; this ends up being very strange food. The best guides were ones that let us have our own preferences for lunches.

We never made it to the silk road; I think it is hard to add when you are trying to see China in three weeks or less. I think the time in Tibet is more critical. If most of your guests have spent enough time in Beijing, Shanghai, and Guillin, you could be able to add Silk Road.