@honzo I assume you resoldered U8900 before attempting all this, since this is a common fault on early 2013 models? If not, I hope you didn't mess up U9100 by reballing...
Haven't got a schematic in front of me at the moment but I believe U8900 sends a GPU good signal to U9100 which will allow U9100 to produce the required signals for backlight and image. If U8900 is not sending the PGOOD signal to U9100 (which happens due to bad solder joints on the chip), there will be no image or backlight on the screen whatsoever.