My family and I did the 72 hour transit in 2013.
This was what we did:
DTW-PEK for 72 hours stand-by
PEK-ICN confirmed tickets on CA
ICN-DTW stand by

If you plan to go PVG-DTW as a nonrev, they will not allow it.
We did all the correct way, and it still caused issues. The gate agents did not want to let us on in DTW, and had to call several supervisors. They also called United to verify our Air China tickets that were UA award tickets.