Hi Stan!
3) From Mac OS 9, run NetBSD's pdisk. I had to use the mac68k version so it would see internal ATA disks (the ppc version only sees SCSI disks); pdisk is available here: https://cdn.netbsd.org/pub/NetBSD/NetBSD-10.0/mac68k/installation/misc/pdisk.sea.hqx In Mac OS 9, pdisk (somewhat unsafely) lets you change (or rename) partitions even if they're active. That can come in handy if you can't boot from a different disk.
After managing to get that utility from the compressed archive, I ran it under Mac OS 9 and changed the partition type back to Apple Bootstrap.
Then I installed Debian from the recommended ISO image for this purpose to the iMac G3 and ... It worked like a charm! Not a single error!
I don't know what kind of black magic you have here, but thank you very much!
Regards,
Manuel Molina