...well...let's call this a "creative pause" ^^:
I followed the FAQ in terms of the black screen on boot-issue and patched MacOS High Sierra on my 11,1 (2009) iMac. So far, so good, everything's fine now in Mac OS and I have a working internal screen!! This is awesome after this adventure.
Here is the last downside:
My Win10 on this machine didn't boot anymore. I tried to boot a Win10 USB-Bootstick, which also didn't start. Meanwhile I deleted the Windows Partition, created a new one and even the Win10 DVD won't boot. So no Win10 Setup at all possible at the moment. The Screen freezes and nothing happens. Is this related to some device-dependent settings, too?