SOLVED - ISSUES WERE CAUSED BY DEFECTIVE MOTHERBOARD
With a replacement motherboard (same hardware revision, same BIOS version) you can just pop the Gigabyte GC-Titan Ridge Thunderbolt AIC into the lowest PCIe slot, connect Thunderbolt, USB 2.0 header and PCIe power delivery cables and it JUST WORKS with the drivers you can download on Gigabyte’s website for the GC-Titan Ridge.
Goddamn, the wasted time.