1. The outer trim pieces are 3 of them, mine are black steel. They cover the edges of the plywood & overlap the carpet. Not sure what else you could use there.
2. The plywood & cargo area floor are two separate pieces. The carpet sits flush with the side panels on mine. Tucking it between the floor & side panels is okay too I think. The chrome trim covers the carpet edges.
3. The rear seatback. The chrome trim needs to fit over the carpet edges. You'll see it when you remove the old piece if it was installed correctly. The seatback latch needs to come off too. Carpet will need to be cut around that opening.
Hope that helps.