High Sierra upgrade - help me, I'm in a death spiral (too many corpses)

After upgrading my laptop, I tried to upgrade my media server (a headless MacMini) to High Sierra.

It's got to the point where it won`t start up any more. Have you managed to recover from this problem?

Problem 1: macOS High Sierra shows "osinstall.mpkg appears to be missing" error.

I downloaded a new copy of the installer and tried again.

Problem 2: macOS High Sierra shows "unable to unmount volume for repair" error message. No longer starts up.

I tried booting in recovery and fixing drive with disk utility (no success - could not unmount internal drive to repair it).

Tried starting in safe mode (no success)

I then tried internet recovery (success). I created a USB install disk for High Sierra and installed to an external drive.

Problem 3: mac will not longer start up - hangs in final part of start up (when booting from the new external drive)

I tried booting to recovery (Command R) to fix the drives (no success, could not unmount internal drive to repair it)

Tried starting in single user made and checked file system (fsck -fy) - file system OK

Reset NVRAM (Command, Option, P, R) - no impact.

Tried starting in safe mode - verbose - many processes appear to have crashed - common messages are "denying page sending SIGKILL", "{XX} process crashed: mdworker - too many corpses being created)

Your help would be most appreciated!