I'm not sure if this will help:
If nothing else, it's reassuring that you're not alone, and also that someone's been looking at it for a while.
I don't know if intel_iommu=off can be make as a kernel option or not. It may be worth a try.
I'm working on a different graphics problem, and when I change configurations and boot I always
- enter kernel parameters from grub, not by editing the grub.conf until I am sure it works; and
- start with runlevel 3, then "startx" once I'm in. That way, if I'm left with a black screen, I can ctrl-alt-backspace and be back in term mode
Is that how you experiment with configurations, or is there a better way?