I don't know why you insist on having a vinyl hump? I eliminated it entirely using three successive layers of plywood and covering it with speaker cloth.
Now the passenger seat goes all the way back.
The hump is the worst original design feature of the car in my view and I have no idea who was the leader of the hump team but maybe he was just a hump himself?
When you really analyze the situation, there is absolutely no reason for that kind of a protrusion. Am I the only one who sees the obvious?