Well, I found a new way to screw the install up.

Freshly flashed N810, installing to internal MMC2.
Files sizes all triple checked.
I put the MMC2 files on a 4 gig card,
stuck it in the MMC1 slot, and tried to install from there.
First file installed fine, made the swap and the image.
Errored out early on the second file.
Log didn't say what it couldn't find.
Your description in the broken installed file, however,
told me something I haven't read anywhere else.

The .BZ2 file has to be on the card you're installing to.

The install was hosed, and the uninstall didn't work.
The internal card was hosed.
Powering up with the USB cable already in beat the locked card problem,
allowing my PC to reformat the borked internal card.
Flash again, copy all four files to MMC2,
and we're off and running.

I've got a good feeling about it this time.