There are only two things I think it could be given your extensive and very well thought out troubleshooting. 1. There is liquid damage that is not seeable. Perhaps on the opposite side of the seeable motherboard or tucked away in a corner. 2. The LCD has gone bad (somehow) and it would need to be replaced. Seeing as that this is a retina MacBook an LCD replacement can be expensive. If you're willing to take on the challenge of taking out your motherboard I would take it out and look for any signs of moisture, especially around the display connector. If you see any, anything at all, take a soft-bristled toothbrush and some Isopropyl alcohol and scrub it clean.

If you have proof of purchase you should be able to take it to a Samsung Authorized Repair shop and they should be able to help you out, or you can call 1-800-SAMSUNG (Samsung Support) and see what they recommend. Though if you do not have proof of purchase they may not be able to help. Good luck!

If you're feeling ambitious you can apply new thermal compound to the processor and heatsink. This may improve cooling. Also while you're in there, double check and make sure there is no dust in any part of the cooling system including fans and fins. Also, be sure to use your laptop on a hard flat surface. Need to make sure your vents have room to pull and exhaust air.