You shouldn't have to be changing a bunch of settings. Maybe reset the bios and start over.
Sometimes you actually try to trouble shoot too much and start getting mixed up on what you did and didn't do. It may be time to start from the beginning.
I have done that 3 times...starting from the beginning, resetting bios and adding cards one by one.
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So I have made a little progress - being a noob I have used youtube as a source to assemble my rig and i did not do what was not shown there - I have not powered mobo with/through a 4 pin molex. As soon as I did it, I could boot the system with all 6 cards. No freeze any more.
...Code 12 error is still there though...