You are able to select the files after really restoring, not just uploading the config backup.
I hope I didn't misunderstand you.
1. You made a config backup and have a backup-config.tar.xz on your nas drive.
2. You made a data backup. To setup a data backup correctly you had to input the hostname and share of your nas drive. The data backup includes a configuration backup like backup-config.tar.xz. The data backup creates duplicity* files. So these files also have to be on your nas.
3. Then something crashed.
4. Now you need a restore.
5. So you take the backup-config.tar.xz. file and restore it. With this backupfile the list of data backups and even the path to your nas drive, which you had to enter when configuring data backup the first time in step 2, should also be restored.
6. Now you should be able to do a data restore and even select what to restore.
Please tell me if I am wrong in some point...