Motherboard issues have been resolved, but not in anything resembling a good outcome.
I bought an ASUS P8Z77-V LE instead. Because I had already attached the heatsink to the processor, I had to wait until I got this kit from Arctic Silver in the mail before I could swap out the boards.
Swapped everything out, and VOILA! I have a bootable system.
Unfortunately, it appears that the hard drive that precipitated this new server build is exhibiting the same behavior. Namely, I can see it in BIOS, and it boots to the “Repair Windows” thingy, but then isn’t recognized as a bootable device. I can access the data if I connect it to the external enclosure I bought, so my next thought is a format /MBR.
If that doesn’t work, then I’ll need to get an SSD.