Whew, I've returned! That office was a hike, but holy cow, those country vets charge a lot less than their city counterparts.
He took a look at my sour crop hen along with another hen who has had trouble gaining weight and currently has an URI. For the sour crop girl, he gave me a broad spectrum antibiotic (SMZ) for her water, told me to massage her crop several times a day (and gave me some tips on how to do it), give her a little olive oil, and said that the nystatin was certainly worth an effort and so to continue it. He said he tended to feel like the bigger problem with sour crops/impacted crops was the obstruction, then the secondary lung infections, rather than the yeast infection, and thus likes to treat them in that order. She did indeed have both yeast and bacteria, though (which he said is usually the case-- that one leads to the other or vice versa). It's worth mentioning that her crop has also hardened quite a bit since I started this post-- it seems to be more impacted now and less water-balloon-like.
I'm so glad I took her in! It was an all day endeavor since I woke up early to get there at 12:01...only to discover they go on lunch from 12:00-2:00.

Killing two hours in a town so small they only have one gas station was tricky, but the vet was a great guy who grew up on a chicken farm and clearly knew his stuff. He also went ahead and gave me a pretty significant about of the SMZ when I told him how far I'd had to drive, so I wouldn't have to return if the problem returned with another chicken.
Any thoughts? I am so, so appreciative to all of you for your help and feedback on this. I've gotten through a lot of chicken issues, but this was a new one to me and I was feeling rather helpless.