@sdetweil - Thanks for the help here and offline. My problem is solved, and for the sake of others following is the resolution.
Problem: MagicMirror install script outputs node.js errors on new RaspbianOS install running under VirtualBox.
Diagnosis [by sdetweil]: “so, the problem is that the intel raspian is a hybrid, x86_64 kernel with a i386 user runtime.
and they stopped making a nodejs for i386 after node 12”
Solution: Rebuilt the virtual machine using Ubuntu instead of RaspbianOS. [I’m sure any Debian or other Linux distribution would work just fine…]
Thanks again - I can now use this as a dev machine and spare myself the risks associated with fiddling with the production machine!