The dud PC has gotten so bad that it can't even fully load the desktop, except in safe mode. A normal boot starts to freeze before the system tray icons load in, and bluescreens into a reboot shortly after. I can't even get it to restore Windows, it just says that something went wrong.
It runs in safe mode, though, seemingly indefinitely. At least that allowed me to check that none of my files remain on the drive (at least not obviously). Maybe I just wasn't spending enough time in safe mode for the instability to hit.
Maybe CyberpowerPC can figure out what component is causing all this havoc, when they try to refurbish it / salvage the parts, but I sure as heck have no idea what caused all this. Temperatures were fine whenever I checked. Chkdsk never showed anything but minor errors, and mostly nothing. Windows Defender never found any infected files. None of my data was lost, except when I wiped it to restore Windows to as pristine a state as possible. I had a couple of long stretches of stability to play Saints Row 2022 and Nobody Saves the World, and Saints Row only crashed on me once or twice (which is actually pretty damn impressive for a game known to be buggy and unpolished).
Probably should have run memtest on this one while it was stable. I did run that on the first dud, but didn't get any errors in the hour that I ran it.
Oh well. Guess I get my internet via my phone and Raspberry Pi for an estimated 5 - 7 days. ($24 for overnight shipping didn't seem like good value.)