You have two phones here since the motherboard *IS* the phone.
Phone 1: uncleaned water damage. "cleaned the motherboard as best I could with 91% alcohol on the display connections." That is pretty much the same as doing nothing. Think of corrosion like a cancer, it will continue to spread until it is removed. There are parts all along the motherboard that have some role in the generation of the display. Corrosion on any one of a number of components will cause an issue like you describe.
Phone 2: From the description of the problem, I would guess that this board had water damage as well. Also possible it that you are putting a working motherboard into a frame with a water damaged LCD.
Solution: Replace LCD. If problem still exists, pursue board-level water damage restoration.