He's still alive. What a tough little guy!
The first 3 pictures are from last night. A big chunk of shell that was barely hanging on when I found him has now come off. (Actually, I think it was just stuck by a bit of dried mucus, and my frequent water mistings made it let go.) But he was cruising around, and he ate a bit of apple while I was watching.
In this picture, taken just minutes ago, he's resting piggyback on the other snail. I have mixed feelings as to whether I should even have another snail in with him. Apart from the silly, sentimental, and probably unfounded notion that he would be happier with a "friend", I have realized that it's good to have a control subject living under the same conditions to compare its behavior with that of the injured one. I would probably worry that my snail is not very active for much of the time, except that the healthy one isn't really doing much either.
But seeing them on top of each other like this, I worry: what if the healthy one did this to the broken one?