Can you get rerouted on a codeshare operated by United? UA allows pets internationally; the fees can get expensive and you have to meet certain criteria, but they certainly allow it as long as the receiving country does.

I'm also surprised that you were told USAir doesn't take pets internationally... their own website says only, "If you are traveling to an international destination, there may be restrictions. Restrictions vary by destination, and customers should contact the appropriate embassy or consulate at least four weeks before the trip." It doesn't say they don't accept pets.

Have you tried calling back and trying a third time?

Welcome to FT; I'm sorry it's under such circumstances!