I am also thinking about using the same video card that you have, the 6870. If I set up my system using integrated intel 4600 graphics, how would I then later install the video card? Would I have to boot into the system and put in the Graphics enabler message then run Multibeast again? Do you know if you can get two displays working off the 6870?
I'm using a 6850 in my Sandy Bridge system. To get dual monitors, I had to use the DVI-D and DisplayPort (to DVI-D smart adapter) connections. According to my 6850 users manual, the two DVI connections can not be used together - dumb! - at least on the 6850.
I was wondering if you had to put in the -v graphicsenabler command on the last try which worked? I noticed that you are using integrated graphics. I assumed that for the B85 chipset you would not have to use this command on installation of Mavericks.
as for the IGFX in the BIOS, I turned it off after putting in the card. But when I was installing 10.8 I had to do it using Intel's integrated graphics, and I remember I didn't mess with GE back then, everything rolled along without a hitch.
I also installed Mavericks on this particular (Haswell) build on IGFX without any need to mess with GE, but that might be due to previous changes in default GE setting in Unibeast/Multibeast.