Ok, that was dumb - I should take a minute to learn about concepts I don't understand! The proper way to get libusb0.dll is to go here
and follow the instructions, which will include booting the Kindle into fastboot mode. There is no actual libusb0.dll file involved, it's installed automatically by the tool they provided. I can post more details if anyone on my beginner level is trying to retrace my steps.
But the upshot is that the windows fastboot tool works, at least a little bit, and at last I am talking to my severely bricked Kindle!! Thanks hostar! fastboot getvar bootmode and getvar serial both work. It at least thinks that setvar bootmode works, although I haven't seen much difference so far in my black screen of death when I reboot with either diags, main or factory. Since you mention flashing partitions doesn't work at the moment, I'll hold off on trying any of those commands until those are ready to go. But pleased by the first progress in a couple of weeks!