Oh, you've been busy! Thank you for the information.
After rebooting with driver signing disabled, I switched my bricked K4NT to fastboot to install the libusb-32 driver through the Device Manager. I hit the "Update Driver" button, and this time it installed almost instantly. I can't find the kindle in the device list, though.
When I run fastboot.exe, it shows me the error:
"The program can't start because libusb0.dll is missing from your computer. Try reinstalling the program to fix this problem."
It looks like my next step is to spend a few hours downloading resources to build me a 32-bit virtual machine. I was hoping I didn't have to.