I'm working from memory here, I wrote this code over 4 years ago, but I did find out there's lots of ways to get a dark panel or other issues you mention. And it's very hard sometimes to get it all right. It's also easy to have it almost working with parameters that are so wrong that getting back to a dark screen is a good sign!
To add to the fun, vendors can ship any number of panels with a given laptop that are very different in behavior. We had to put 4 panel types into the falco configuration and try to figure out which one it was.
One thing you can try that sometimes works, if you have serial, is to do NO video initialization at all and let X11 do it instead. I did a lot of debug this way and on ivybridge it works pretty well. You can take the normal boot path and, again assuming you have serial, start Xorg and just see if you get a lightup. If so, well, you've got a starting point: you can put debug prints in the driver to show each and everything it does to the panel, and compare to coreboot. There may be a surprise in there somewhere. The timing is extremely tricky sometimes.
You could also consider running the vga bios in the 'safe' mode (i.e. the emulator in coreboot) and tracing all the IOs. Sometimes you can learn something that way.
I also wonder if you could use the code Nico put in for the graphics init. I have no idea if it's possible, Nico could tell us. But that code looks really thorough and might work better.
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ron minnich