Just wanted to follow up. I put the duck back with the flock since her attitude was good. It took about a month but she finally started laying eggs again! Then she disappeared and I thought she was dead! She reappeared to eat and take a bath, so had been hiding out, and she's been at that for about 6 weeks, going off to bed down at some hidden spot and then coming back to take baths and eat periodically. I thought she was sitting on a nest but incubation time came and went and she is still at it. But, she's happy and quacking when she rejoins the flock at these times. I'd force her to go to bed with the rest in their run at night but if she's working on a nest I don't want to disrupt that. Thankfully, we seem to be pretty free of predators.
Anyway, the weird stricture seemed to resolve, and though she had that egg stuck in her for quite some time, she began laying again! She was squirting diarrhea, though, and I could always tell which egg was hers because it had diarrhea on it! Strangest thing.