I’ve finally removed my old photo gallery (established in 2001) and everything is now in one structure of Gallery2. 2901 pictures contained within 162 albums has been moved in 20 hours of sorting and guessing where and when. Then it took my server 6 minutes to remove all old content from the disk array (simply executing shell command
rm -rf obsoleted). Now I have more than 20 GB of new free space.
Some dates and places in the Gallery2 titles and descriptions are probably incorrect, so please tell me if you find something to make clear.