Before trying what you have suggested, I have installed the package xterm in my container. Then within this xterm I tried to locate the stuck installation process with ps although I have no idea if it should be visible from such a terminal or not. But ps wouldn't run. I got a message asking me to try "mount -t proc /proc" which I did only to be told that the "-t" option could only be run as root. But I was refused to use both sudo and su.
So I tried your solution. However there was no .json nor any .xml file within any directory. The command
# find / -iname "libertine" -print | egrep 'json|xml'
did not yield any result.
I searched for a root ~/.cache/libertine-container/CONTAINER-IDENTIFIER/rootfs/ directory but couldn't find any. In fact, there is no .cache directory under ~root nor even under "/". There is a "cache" directory (with no dot) under "/" but it has nothing relevant inside.
However I did find the container directory ~phablet/.local/share/libertine-container/user-data/xenial. So I deleted this entire directory.
Then I rebooted the device and when the Ubuntu logo appeared and I swiped sideways, no on-screen keyboard appeared to enter my pass code! I had to connect a mechanical keyboard through the USB port to do so... Furthermore, the container still exists and the stuck installation process subsists with its silly blue spinning circle!!! Arghhhhh
I think I will end up reinstalling the whole operating system.