Update: Problem solved. After consulting a repair tech, he informed me that the part of the cable closest to the screen is very fragile. He continued to add that there's a piece of "glass" or similar inside the cable and it's likely that when I was feeding the cables through the phone body to set the screen in that I may have bent that part of the cable too far thus damaging the connection. (there was no obvious way too see that any damage occurred) This actually made a lot of sense though - cause when I tried reinstalling my previous cracked screen I ended up with the same issue. Which led me to believe that I also bent that cable too far l when installing the screen. (And I was trying to be so careful.. just didn't realize that cable was soooo fragile!)
I got a new screen and was extra careful not to move that cable too much at the base. New screen worked great. Live and Learn.