Oh, the backside of the seatback is designed funny. . . To remove the cover, ideally you need to remove the headrest adjuster/inserts (whatever those things are called) before you can pull the cover off but unfortunately to access those, the cover needs to be removed.
If it's possible to remove these inserts before the seatback cover, I haven't found a way. What I ended up doing was bending the spring loaded lever until there was enough room to pull the cover off.