There are a few things I would try. Not sure what you meant by iTunes thing. 1. Make sure you have the latest version of iTunes installed and a good lightning cable. 2. Download (3UTools) and try restoring or updating. 3. Put your phone in DFU mode and try restoring with iTunes. Let us know the outcome.
Greyed out Wi-Fi can be caused by two things. 1. No power getting to Wi-Fi chip. 2. Wi-Fi chip is damaged. I would guarantee your Wi-Fi chip is damaged and will need to be replaced or re-balled. On an IPhone 6 the repair cost would be probably more than your phone is worth. Fonez.ie do not offer micro soldering. You will need to find a phone repairer who does micro soldering and even then they might not do this type of repair as the Wi-Fi chip has under-fill and it is common to pull pads while removing. Most phone repairers have quote forms on their website. Send off a few quote forms for replacing the Wi-Fi chip.
If you had long screw damage it would affect the image not the backlight. More than likely you have nudged a filter near the FPC. You will need a multimeter and the backlight diagram to work out what component on the backlight circuit is bad.
This is becoming a common problem on the IPhone 8. It is the FPC on the board that is causing the problem. Sometimes you can manipulate it to make the battery FPC fit snug but other times you need to replace it.
I am not sure what equipment you have to diagnose the problem. Assuming you have the bare minimum. 1. It most likely has a short so we need to find what power rail. 2. Find the schematics for the phone. 3. Remove the board from the housing. 4. Start diode mode testing with your multimeter. Compare readings with a good phone and let us know what you find.
The NAND is tied to the CPU, Baseband and some other IC’s. You would need to put the NAND back in the same board and get it to boot to retrieve data. You cannot retrieve data by extracting it from the NAND. The state the NANDS in it would be extremely difficult and time consuming but I wouldn’t say it is impossible. What are the pads on the logic board like?
This would have nothing to do with the battery. You most likely have an open line on one of your camera power rails. You would need to diode the camera FPC’s and compare readings to a known good board or board view software / schematics. That will tell you where the problem is.