Thank you.
I will now try and merge all available backups of the original source folder into one (whereby I may have to remove file name extensions made by Syncthing according to the pattern ~[modification date] from the file name first).
Then I shall compare this merged backup with the current state of the source folder to see if and where mass deletions have occurred.
Man how I wished ST had asked me something like
dude I am about to delete 80% of your share, is that ok with you?