I'm still trying to get this sorted. During install I never get the second part of the install where you boot "Install macOS"; the installer just ends up rebooting back to 17A365 after the first part completes. From what I'm seeing it really feels like (for some reason) the installer bails-out on the whole thing and just reboots before creating the "Install macOS" boot disk.
Could one of you resident geniuses take a look at the attached install log file? This is the install.log from the time that I opened the downloaded supplemental installer (linked in OP) to when I finished rebooting back into 17A365. Yeah, it's long. I read through the entire thing and, unfortunately, I'm not sure what's supposed to be there and what's not supposed to be there. For instance, I'm seeing stuff about FirmwareUpdateLauncher, but I don't know if that's OK or not.
Any pointers to what looks wrong in the log would be helpful.