Hey, I’m actually having a very similar issue, actually almost identical. I tried what you’ve outlined above, however get a black screen (with backlight on) and no post after replacing the two section with FFs and reflashing. I think I may be hitting code I’m not supposed to? USBs are back to working though with the edits.
I have a post here, if you wouldn’t mind helping me out: Issue with AMIBCP saves not posting - #2 by MeatWar