Make sure to use the mechanical shutter. Using a flash is not available with electronic shutter. Perhaps this is the reason for the problem. Another could be contacting problems between camera and flash. Check the contacts and if the flash is seated well.
In my experience the battery indicator does not work correctly with some non-genuine batteries. You could test how long the camera will actually work with one bar battery indicator, the battery might be fully loaded.
Sorry for my misunderstanding.
You could try to take out the battery for a longer period of time, keeping the power switch in the on position. Or try a factory reset.
Welcome to our forum. There are different settings influencing the preview, take a look here
https://fujifilm-dsc.com/en/manual/x-t2/menu_setup/screen_set-up/index.html
Welcome to our forum. The default setting of the display mode results in a brightened prewiew. You have to wath the exposure indicator for correct exposure.