OK, here is some follow up:
I managed to get another 27" 2011 iMac and swapped displays - mine works fine in other iMac, the other one's doesn't in my iMac. This means the LCD and cables are fine. Of course there still may be some problem with my other internals, but...
I managed to get to my iMac using VNC and I see the GPU reported by the system, but the only display available is the one used by VNC. I did also the test with external display and none of the two ports produce any video signal. Also the external display does not show up in the system report.
Let me remind you that my screen got completely black after about 30 minutes after the first iMac startup with flashed K2100 but no OpenCore yet. The computer had the display turned off by the OSX power management, but the backlight was on (there was nothing on the screen but the backlight was clearly visible). I pressed the key to wake the screen up and then the backlight gone. iMac was still working, it didn't shut down or hang. I was also not moving or touching iMac, so it's unlikely something got loose inside for instance...
My question to you is then - is it possible that the reason for the LED #4 is off, is the malfunction of the video card? If this K2100 was flashed ok and is visible to OSX, and the LED #3 is on, but there is no video on external display isn't it pointing to the GPU problem? Is there any other way I could test this GPU to confirm if its ok or not, other than trying to replace it with another one?
Thanks in advance.