This morning, I erased the destination volume using Disk Utility and then downloaded and installed dev beta 9 on my 2013 Mac Pro with external monitor, and beta 9 is now working fine for me.

You have now noted that you were trying to use Accessibility Inspector to explore UI elements in the simulator window. Let me note now that I was using it to explore UI elements in macOS application windows. So perhaps the problem lies with the simulator.