Take a look at the guide rail pieces that you fished out. See if you can identify the one that was originally on the upper inside. That would be the one that "rode" through the sprocket, if it did at all. There may be long wear marks, but look for sprocket marks. Just a shot. If you don't see any, your problem may be a real stretched chain that started slapping worse when it broke the guide rails.

In any case, I agree with the above recommendation of replacing the chain first, and see the situation. Good luck and let us know what you find!