Further update on my issue:
I did a fresh install of Linux Mint 19.1 Mate as guest with Virtualbox 6.0.0 on the same macOS 10.14.4 host as above, updated the LM 19.1 OS files, and applied the 6.0.0 guest additions. calibre 3.40.1 was installed as above using:
sudo -v && wget -nv -O-
https://download.calibre-ebook.com/linux-installer.sh | sudo sh /dev/stdin
calibre launched with a warning about GTK libraries:
stewart@stewart-VB-LM191:~$ calibre
QGtkStyle could not resolve GTK. Make sure you have installed the proper libraries.
QGtkStyle could not resolve GTK. Make sure you have installed the proper libraries.
stewart@stewart-VB-LM191:~$
I did not notice any further issues with calibre.