Only extremely circumstantial evidence. Some guesses about which species died off vs being protected, some climate change signatures that are consistent with a high UV burst.

About the best you can say for it is "you can't prove it didn't happen". Which, given that it was half a billion years ago, may be the best you can get. There isn't a real smoking gun.

The leading hypothesis is more that a long-term climate change caused secondary effects in a vicious cycle. We know the climate change was happening; there's very strong evidence. But it's not clear exactly what caused it or exactly how it led to mass extinction.

No, and the idea is widely derided by paleontologists because it's pretty much untestable.