So I'm happy to report that the clean install of 7.5.5 went without a hitch.
I zapped the HD, installed 7.5.3, updated to 7.5.5. Installed Stuffit Expander then Open Transport 1.1.2.
I moved the "Ethernet (Built-in)" extension into the "Extensions (Disabled)" folder, dropped your driver into Extensions, and rebooted. When the system was loading it when through an unexpected reboot (I think this is where I was supposed to get a System Error) but upon going through the motions again it booted fine.
I opened the TCP/IP control panel, selected "Alternate Ethernet" as the NIC, installed iCab, and presto, I was online.