Result: the hanger *was* bent. I used Dave Moulton's method as Neal suggested, and sure enough, the bottom of the hanger was bent toward the centerline. I got it straightened up, put the derailleur back on, and tested the shifting on the bike stand (it's raining outside). Shifting is much smoother, and so is trimming. Also, both jockey wheels are now right in line directly beneath whichever sprocket the chain is in. Still, it couldn't hurt to start developing a stock of replacement parts, so I'' keep an eye out for some of the derailleurs you guys suggested.

Thanks everyone!
-Eric