Adoptable Storage drives me mad - indicated free/opcupied space does not make sense

Help Fairphone 2
,
6

UPDATE***

  1. new SD card will arrive on Wednesday

  2. Backup: I found
    Backup Tools for FP OS
    Creating a backup of user data using TWRP community edition
    Beste Backup-Lösung für Open OS?
    so I would say that only ADB allows me to backup app data on per-app basis. Is there any guide how to that easily?