Allowing the install to complete before cancelling the script helps with a successful installation.
Having left the install to run past the stages that look like it has crashed. Gave the installation script chance to complete.
I’ve successfully installed Magicmirror using the latest 2017-09-07 Raspbian stretch. Following these procedures mentioned above, nb as user ‘pi’ not ‘root’.
Hope this helps some other people.
Thanks!