You definitely have to get rid of Multiprocessing folder! The stupid thing is that you can't disable contents of MP folder with "Extensions off" (shift key), you have to remove manually while booted from another drive/volume.
I don't have PB 5,6 but I do have 12" PB 6,8 which is also 7447a based and I was able to boot it succesfully into OS9 by first using instructions by Greystash and later by using ROM file by Nanopico (I think, I modified it slightly) + only the Screamer fix in OF.
That tip about "Extensions Off" Shift key boot not being sufficient to prevent the MP Extensions from loading is very useful, thank you. I will give this another go, now with that in mind. I may also try that ROM you shared. I forgot to mention before, but the outcome I got each time happened regardless if I was holding Shift or not when booting. I will check the contents of the CD I was using to boot with to see if the MP Extensions were there or not.
Unlike Greystash's, my model is the
PB5,8 rather than PB5,
6 though, so there are some things in hardware and firmware that set my model apart from the PBs that successfully booted so far (DDR2 RAM, GPT booting support IIRC etc.). Still, I will keep trying. I wish I knew how to disable a given OF device tree item, though.
About 7447a-based PBs having reduced speed in OS 9, that's the first time I heard about such a thing, I have to admit I'm skeptical about that. I do think, though, that I have seen some discussions in the past on PPC MacRumors subforum darthnvader talking about configuring in OF processor states that allow us to underclock the PPC CPUs by setting them to some sort of "extra low power" state or the other, but that was years ago, so my memory is fuzzy on that.