Is it possible to restore some files from a backup located on FTP repository without downloading the archive onto the server with Plesk?
At the moment, such functionality is not available in Plesk.
There is a request that can be voted for on UserVoice: https://plesk.uservoice.com/forums/184549-feature-suggestions/suggestions/15852874-backup-and-restoration
Most popular suggestions will be implemented in future releases.
As a workaround you may use the following scenarios:
1. Create a backup for each subscription separately.
2. Mount network share (e.g. NFS) to your server and store backups there. In that case Plesk would store backup files separately per subscription.