And the prize for most thorough newbie diagnostic before crying halp goes to ...

I have no further suggestions than what you've come up with yourself, and what others have shared here. You should be able to find a boot/install CD of System 7.6.1 or Mac OS 8.1 on ebay, or as suggested, *ahem* elsewhere. If that fails to boot with proper termination, then as you've already deduced I think a Nubus SCSI card is your next step. That will give you faster SCSI performance as well.