Today I got the bright idea to talk to a co-worker who is the travel coordinator at my company. She used to work for USAirways and has all kinds of inside info and access. I gave her my record number, and here's what she found when she called her contact at USAirways:
When I made my seat selections with Orbitz in November, they sent them to USAirways as a "request." Normally, USAirways sends back a "confirm" message to Orbitz. This time they did not, and in the intervening time there was an equipment (airplane, that is) change that would have invalidated all seat choices anyway. So, in her opinion, the problem was indeed Orbitz' fault, not USAirways.
She said calling USAirways at this point will not accomplish anything more. Her advice is to check the USAirways website periodically and see if any seats together open up. If not, then I should arrive early (sigh) and talk to the people at check-in and at the gate, as well as ask to be directed to Special Services at our connecting city.
Grump. Well, at least now I know what happened and what my options are.
cvirtue's suggestion that I put the kids in T-shirts reading "Orbitz messed up my seat selection" is looking more and more appealing.