What happens when you boot the installer with -v?
I have an update, not sure what to make of this.
I pulled all my drives, the 970 and started a fresh install of 10.10, got that running(intel 4600 only). Updated 10.10.0-10.10.5(14F1509). I installed 346.02.03f04, installed the 970, crossed my fingers and loaded the web drivers. For the first time in months the thing booted without crashes. I did not have any additional monitors attached at the time. System reports both cards in system profiler, but only one in about this mac.
I shut it down, attached my monitors and booted. Same kernel panic. Problem not solved.
I've run some tests and heres some more information, maybe someone will have some insights.
Working conditions of the GTX970 are far from acceptable. It can be booted with the 4600 disabled in bios(I need a DVI connection from it), if only 1 display is attached to either Port 2(dvi-d) or Port 3(Hdmi) maybe the display port but I don't have a monitor to test. You cannot attach any monitor to Port 1(dvi-i) during boot without failure, both a div-i and dvi-d device was tested. It is also not possible to boot while more than one device is installed.
Interestingly enough, since OSX can sense an attached display and add it while running, plugging in all my monitors while the system is up results in them working.
It is also impossible to boot while both the 970 and the 4600 are active. The most successful attempt brought me to the login, glitchy and only displaying on the 4600. It was also possible to boot the 970 with a single display while the 4600 was enabled but not injected, that did cause the issue of it not working.
I know there are some bios updates available for the 970 I'm using, but I have no idea if it will make this worse or not. Not sure that if I upgrade them if I can roll them back. Outside of doing that all I can think of is rolling back to 10.10.0 but I don't know if it will work. I didn't acquire this card until my system was at 10.10.2 and had everything working stable on a 10.10.3 build. I have no idea how difficult it would be to find those updates.
Side Note: I attempted to load the 10.11 installer when the GPU and drived where pulled. Still did nothing, verbose gets 2 lines in and crashes. optiofix starts overrides ,using reloc block and then panics. I gave up on that.