I was nearly denied boarding by ANA in Seattle because the agents there didn't know the transit without visa rules. Only after a stressful 90 minute period of showing them the rules on the Chinese embassy website did they reluctantly let me board the flight to NRT. They were still skeptical and made sure I was flagged upon arrival in NRT so I could not continue on my connection to PVG without another document check. Luckily the staff in NRT were much more familiar with the rules and there were no further issues.
The point is, what you are doing is perfectly allowed but agents are not trained properly regarding all of the visa rules and may not look up the rules correctly in the system, may not believe you, etc. I would get to the airport earlier than normal and be prepared to have slow downs any time anyone wants to see an ID and/or visa (particularly at check in and at the gate). By the way, I did call UA in the middle of my ordeal with ANA (ticket booked with MP miles) and the 1K desk was no help either - they also incorrectly asserted that I needed a visa.
I would be sure to have a copy of the Chinese embassy's website handy and also to tell them to keep looking in their system if at first they try to deny you. I think in my case they were looking for the word "stopover" or something similar, when the proper nomenclature was "transit without visa" or something along those lines. The poor English skills of the ANA agents in SEA certainly didn't help.