My x-t5 does not exhibit the focusing switch behaviour as you report it, so that is very strange and indicative of a fault. It does not matter whether the flash is attached or not. Once you set the camera for your studio flash, say 1/250th at f5.6, the camera, which is showing you what you will get at that exposure without the flash, will show a black screen unless the ambient light is brighter than what you would typically get indoors. That is why, as Jerry says, you have to set preview exp/wb to off. I have set a button for this.
I connected to FRAME.IO a while back and it works fine, but the camera wouldn't connect to the internet all of a sudden today and would get stuck on the reset screen, including initializing and even switching USB Connection mode. Is anyone else experiencing the same thing?