To remove the back seat bottom I do what I saw my Dad do back in the 60's....
One side at a time sit on the seat, grasp the front edge & extend your legs pushing straight back. The seat will slide back a bit, at that moment raise your butt along with the front edge & the seat is free of the hook/clip (whatever you want to call it) slide it forward a bit to remove after both sides are free.
This has worked on every car I've ever tried it on.