Once again, please check the menu version of the ACA500plus. This may be the reason why the ACA1221 does not start.
Is there any way to download a pre-made image of a working workbench that I can apply to one the CF cards from another computer to see if the system can at least boot into Workbench via CF card (vs a problem with the auto installation process)?
I can't offer that, but the automatic installer will only see a CF card in the boot slot if you insert an empty card before power-up. This is because the boot slot is not hot-plug compatible - only the aux slot is. With an empty card in the boot slot at power-up, you should be able to launch the automatic installer and make a bootable WB that way.
Checking oyur earlier post, you seem to attempt to launch the installer with F1 or F2 options. The installer is in the Installer sub-menu: Press F7, then F1 - this will launch the automatic installer. Once again, the card must be inserted into the boot slot before powering up, otherwise it will not work.