If the backrest latch is "stuck" or not releasing you can manually release the catch itself.
Grab a flat bladed screwdriver.
Laying in the cargo area, facing the stuck seat back you should see the latch (attached to the seat) surrounding a hoop attached to the car body at the front edge of the cargo shelf near the rear door.
At the upper corner close on the seat side of the catch you should be able to take the flat bladed screwdriver and raise the tab which positively locks the catch when closed.
You will need to raise it about 10mm or 3/8 inch to release the lock.
Rob