I did eventually discover the solution to this problem, and it was unexpected.
I bought a second identical unit that was working and systematically began swapping out parts until I found the one that was faulty.
It turns out it was the graphics card, which is interesting because the symptoms and trouble shooting suggested it was not. An external monitor would work just fine, and the image was visible on the main screen when a flashlight was shined at an angle on the screen. So apparently the damaged part of the graphics card was just the part responsible for powering the LED backlight.
I removed and cleaned the card, and reflowed it by baking it in the oven. The unit now works perfectly. I know and understand that this may be a temporary fix, but it’s worked for the past month and if it does eventually fail I can always just replace the card.