Well it looks like this is finally resolved. I had to play telephone-agent roulette with Expedia before I found someone who understood the situation. This took hours, in fact, because of the hold times.

Fortunately, after speaking with three people at Expedia, Expedia is going to give me a full credit for my fare and it can be used on any future itineraries booked on US through Expedia for up to one year. If I book a trip on Expedia that is cheaper than my credit I can use the remainder of my credit for another US flight booked through Expedia. I had Expedia send me an email explaining these terms, so I have it in writing, too. ^ ^ ^

US, on the other hand, would still force me to book any future itinerary on US, Air Mexico, and Mexicana if I were to try to use my credit directly through US. The US phone supervisor accused Expedia of giving false info to customers "all the time".