Trust me when I tell you that tufting is immensely easier using foam for the base. I would use 2 1/2" or 3" thick foam in the firmer range from 45 to 65. Then what you have to do is cut the foam to the size of the piece you're working on and mark out the hole pattern you decide to use on the foam.
The basic concept of tufting is that there is a mathematical correlation between the holes in the foam and where the buttons go through the leather. The correlation is based on two factors, the thickness of the foam and the spacing of the holes in the foam. Where the buttons go through the leather would be larger than the holes in the foam and deck pad. That correlation stays the same for the entire piece, it never changes. You don't tuft the piece right to the furniture frame, you would tuft the leather through the foam and attach it to a deck pad that has the same hole spacing marked on it as the foam. That whole assembly would then go over the springs. You don't attach that deck pad, foam, leather assembly to the furniture until after it has been tufted. This way you can do your tufting on a flat surface instead of right to the furniture.
The old leather will not do you any good unless you can duplicate the tufting exactly as it was done in the first place, including the filling. Because this would be next to impossible, it would be easier to start from scratch. What would happen is that you would try to duplicate it and then have to do it over and over and still probably not have it come out right.
There is no way to find a hide that is as long and as wide as you need. That in itself is not an easy proposition. In order to tuft a large piece, two or more pieces of leather would have to be cut in a zig zag pattern and then sewn together. The seams would be hidden by the folds in the tufts.
No one lives forever, the trick is creating something that will.