flychina.com is a booking engine -- and possibly a consolidator, although when I checked one fare, it just gave the same results as kayak.com, which gives published fares mostly.

Indeed, flychina probably won't or can't do anything for you, but that doesn't rule out going back to Northwest. Tell NW what you want and explain that flychina can't help, maybe they can take over the booking. When an agent tells you it's impossible, you then put into play the NEXT key element of the Flyertalk gospel: thank them politely, hang up, then call back and try again.

If you don't have elite status with NW/DL, you may have to try multiple times with long waits to get a human each time. Even then it might not work but usually, when what you are trying to do is give more money to the airline to get a stopover, you will eventually find someone who will work with you.