Several answers here, just to clear it up: This is to do with apple’s serialisation as Alfy suggested. There are some models where screen replacements work fine (if a good quality screen is bought) but in my experience the ambient light sensor is almost always serialised so keyboard backlight is either disabled entirely because of the sensor not registering as the original or you turn off auto brightness and manually control screen and KBD backlight. Same for True Tone. as Alfy also said it is paired through the camera so you would have to transfer the camera flex from old to new screen but bear in mind replacing LCD only is not an easy repair by any means

Sounds like a backlight issue. 9/10 times it will be the display that needs replacing. However if unresolved then it will mean the logic board will need fixing

Sounds strange. Could be that your software is not working very well. You could try a reset of all settings or restoring the unit as new to test. Furthermore check the power button is not sticky or held down slightly. If it is use an alcoholic wipe to loosen it up.

Ex-Genius here... Apple replace the entire display for Touch ID related issues. They use a "calibration" machine that pairs the new Touch ID to the logic board processor.

Could be that the dock is full of fluff. The Apple Store will clean it free of charge

If your machine is a retina MacBook there is no hard drive cable. It's an SSD that slots into the logic board. Likelihood is that the disk needs repartitioning and a new OS installing. In some cases Apple use "flash storage repair" diagnostics at the Genius Bar. Maybe worth checking your one does/doesn't need it

Yes should work absolutely fine

Sits on the top edge of the logic board and pushing against the housing when you slide the board in. Top left hand side I believe

Luckily the iPhone 6s is very water resistant. The light spots will be where the water has got inbetween the display. A new display will probably resolve it

Sounds like potential garbled video/distortion. Could try a 2 button reset and see if it improves. From working on the Genius Bar, it's progressive and results in a unit replacement. So could need a logic board replacement worst case