One minor correction:
The Downgrader is not threaded, so anywhere during the time it says "Downloading", it hasn't actually installed anything.
There was a user whose Wii got stuck during an IOS change -- namely during the time it said "Downloading/Installing - XX%". I told him he would be fine restarting his Wii, and surely enough, it booted with no problem.
Also, keep in mind that this software is still based on the cIOS/Wad Installer, which require that a full WAD be present. If it's partial, it can't verify the files and thus won't install. So, even if it were threaded, it wouldn't be installing a partial WAD.