They've returned from their trip so I have an update.

The Continental agent in Nashville would only put the bags through to Shanghai because that's all he could see on his screen. You can see how that would make sense. Otherwise anyone could show up with a fake itinerary and ask that bags be sent anywhere.

In Newark, they were upgraded to business class for the flight to Shanghai! The gate agent said it was because they had purchased their tickets recently and paid a higher price than most people. As expected, they said it was f.a.b.u.l.o.u.s.

After (easily) clearing customs in Shanghai, the Xiamen Air agent would not take the bags because it was 7 hours until the flight to Fuzhou. He said there would be an overage charge when they returned but was vague on the cost.

The group (10 people) left their bags with an in-airport storage company. The price was based on size. Most of the suitcases were too big to fit into the lockers the company used so they were stacked in a back room. H said it looked very non-secure. It was 300RMB for 16 pieces for 0-4 hours.

When the 10 of them checked in for the flight, all of the suitcases were checked in with no comment. H asked if there was a charge and the man waved them away. H's Mandarin is very basic and the agent's English was even less.

On the way home, they flew Fuzhou to Beijing and were charged a weight overage fee of 23RMB/kilo over the allowed 20/kg/person.

Thank you for your help. I hope my details can help someone in the future.