The red glow is the proximity sensor. In certain low light situations you may notice the red glow. It's no big deal. Normal functionality. Some are more noticeable that others. There is no such thing as a water detection light that turns on when it's wet...
Normally where there is a bright spot (or dull color) there is pressure on the LCD in that location. Double check cable connections and that screws are secure. Also if there is any resistance at all when putting it back together, something is out of position. The screen should slide and click easily in place.
Whole Logic Board replacement would be needed. If you shorted out soldering points on the MLB, it probably shorted out most of the board. Hard to tell where the current slowed down and stopped or the path it took.
Touch ID is specific to the phone and can only be replaced by Apple. The Touch ID and the Logic Board are paired together via Apple Firmware. The phone needs to be completely replaced.
Unplug from power. Hold the power button down for 10 seconds. Plug back into power. Attempt to turn on. High fan RPM is usually an SMC issue. If the first option doesn't work recheck all your cables. Then try to clear the NVRAM (Cmd+Option+P+R) before the gray screen. Should get 2 boot chimes then release. Reply back if you're still having issues after that.
Yes you can. Also you can always check on Kingston's website. It lists what the max memory is by machine selection. http://www.kingston.com/us/memory/search...