Just a dead lcd on the main display. The fact that you are able to view on a separate display means your video card / motherboard are working fine. Just need to replace the main lcd. no need to over complicate it :)
sounds like you have a bad connection or missing connection. Go through and double check to make sure all cables are attached to the mother board correctly. did you take out the battery and ac adapter before doing the keyboard? if not it could cause a short on the logic board.
the prox sensor is apart of the UI flex (power flex). can find it here: https://www.mobiledefenders.com/power-volume-flex-cable-for-lg-g4.html on mobile defenders (trust them more then ebay sellers by far.)