great work with the experimentation. Very thorough. I suppose the only glaring difference is the pixel depth changing, and the automatically adjust brightness changing. I wonder if High Sierra is not reading the EDID from the screen. I need to look into it more and see why this may be, and how to determine for sure.
There are many complaints in other forums about brightness and sleep not working on other models (completely unmodified), so they changed something for sure.
Just to be clear, your system profiler no longer shows the AMD GPU at all correct?
Have you tried bridging r8911 back over and reseting NVRAM to clear the EFI work? I am just curious if functionality is restored, or if we are chasing our tails on an unrelated bug.