First of all, to ensure no interference, remove the board and power supply from the chassis and test it on a grounded/isolated surface with only one DIMM installed (in slot 0) and only power supply and keyboard connected to it. Yes, it's a pain; do it anyway. After power on, use F2 to enter BIOS Setup and then use F9 to set to the BIOS configuration to defaults. Yes, you've probably done this before; do it again anyway. Plug a USB 2.0 (not 3.0!) flash drive that has been freshly formatted (yes, do it again!) with FAT32 and containing only the 48.BIO file, into one of the USB 2.0 port on the rear of the board (not into USB 3.0 and not into any cabled ports or hubs). Save settings and exit BIOS setup. When BIOS splash screen appears, press F7. You should get a dialog that allows you to browse to and select the .BIO file on this flash drive. Telling it to proceed, what happens? Be explicit in your details. Detail all messages seen.
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I don't have an exact step that fixed the issue, but after multiple attempts and reattempts, I was able to get the BIOS updated all the way to 120 (latest). I had to run the recovery process a few times, and iFlash2 a few other times, but eventually the system decided to take the latest BIOS and allow me to boot from USB. I was able to install Windows 7, activate it, and then upgrade to Windows 10. So far so good, and it seems perfectly stable now.
Thank you so much for all the advice. Much appreciated!