Any progress with this investigation?
I have a late 2011 17″ MacBook Pro which I modded according to your instructions when it started having GPU issues. Everything worked great with Sierra, but after the High Sierra upgrade it has the same symptoms: backlight brightness adjustment doesn’t work and the display never wakes up after sleep mode. I tried replacing the AppleBacklight.kext with the one from Sierra, but that didn’t do anything.
There is a Time Machine backup of the machine from the time before the upgrade, so restoring to it is possible. Any better ideas? Anything I could help with?