I'm not familiar with configuration like this but I would suugest that beside configuring the event-options policy trap , you can configure :
set system archival configuration transfer-on-commit archive-sites ftp://username@destination_ftp_ip_address/foldername password <password>
So you can : 1- backup your configuration on every change 2- from that data the file was created on your ftp server and the date event trap was sent , you can know who made the changes ..