I think (hope) Misty has finally taken a turn for the better. Her symptoms are slowly disappearing, and she is returning to her normal habits. I am keeping my fingers crossed that we don't have any more setbacks. Hope I'm not jinxing myself by posting this!
Since this was such a resistant bug, my vet is keeping Misty on the antibiotics for at least another two weeks. Plus, she has one more drug up her sleeve, if this one doesn't do the trick.