Yes your are right, /usr/share/piwigo is meant to contain read-only files and dirs

When you launch "piwigo-instance-manager create g1", it creates the writeable directories in /var/piwigo/g1 (_data, upload, plugins, ...) and symlinks to /usr/share/piwigo for read-only files and dirs (using stow)

So for piwigo g1 instance, PHPWG_ROOT_PATH is /var/piwigo/g1

Because I don't know piwigo very well, (I am not able to test quickly if there is broken feature in my package), there maybe some directory that should exist as real directory in /var/piwigo/g1 instead of symlink (like I did for plugins directory).

In fact the only knowledge in the package is to be able to split in 4 parts all files of piwigo

1) read-only files and directory (like the include directory)

2) directory writeable with mix of read-only directories (like the plugins dir that contains already a few plugins in the tarball) and writable directories (from the admin console we download plugin as writeable directory in the plugins directory)
2.1) writeable directory for data (to put in /var/piwigo/g1)
2.2) writeable directory for configuration (to put in /etc/piwigo/g1)

3) directory writeable with only writeable files and dirs (like the _data directory)

To remove completely the package and their data from your debian,

you lists the instances created with piwigo-instance-manager

piwigo-instance-manager list

you delete them with

piwigo-instance-manager delete g1

you remove the piwigo package

dpkg -r piwigo

Note that no file from piwigo in the package has been modified. Can you tell me where I can read about webapp policy? I should change /var/piwigo into /var/lib/piwigo (to better match FHS policy)

Because I am currently testing my setup, I do small improvements in the package, if you re download it, the md5 may have changed.