WE FOUND SONYA!!!!
She was found under a porch on a cold and rainy night a couple weeks ago by some people who own a local business, and they had been keeping her there ever since. When B was out looking for Olwen a couple nights ago, he talked to several people and one of them knocked on our door last night to tell us that there were signs up about a grey cat that had been found. When we initially looked at the signs, our hearts sank because they said it was a male cat. But we decided to stop by and look anyway, and fortunately for us, the people keeping her are incompetent at sexing cats.
So now she's back home, skinny but sweet as ever. The people who found her had been getting attached to her (and who wouldn't, with that lovey personality and thunderous purr?), so I know she would have had a good home if we hadn't reclaimed her. I had really given up hope by this point and had reduced myself to hoping for that. But oh, I'm more happy than I can say that my cuddly girl is back home.