Can't Interact with Second External Monitor
You could try using the program I wrote, displayplacer, which was developed to deal with the wonkiness of macOS screen layouts. For your particular situation, you'll want to run displayplcer list to output your current monitor setup. Then, edit that profile so that it isn't mirroring. Mirrored monitors are after the + sign in the id section. It'll take a bit of fiddling. Once you get it working though, you can assign that displayplacer command to a hotkey. Then, whenever this happens again (usually every time you plug in), type the hotkey to set your monitors to the correct layout.