About that vert seat back. the frame is curved. The upholstery should be curved only at the 2 end panels. The are where the human back meets it should be straight. Actually, it should look a lot like any other rear seat back only with curved extensiona on the ends. As it is right now the 2 rear passengers would be turned toward each other.
I have to ask.., Did you by chance remove the original seat covers before going to the installer ? A lot of people do that. They throw away rotten, mildewed batting and expect the installer to know what filler depth and shape is required in the different areas.
These seams marked are where the transition from raised and flat to curved should be.
Some skins have pockets for listing rods in areas like this to dram the skin and create shape.