I've never done tufting (or a headboard), but have reupholstered chairs & sofas.

I think you could temporarily tack down the top & bottom so you see how much you have to work with. When I needed to temporarily tack, I did not press the nose of the staple gun firmly against the area to be tacked. That way, the staple did not go all the way in and it was easier to remove later on.

If it gains you some fabric, forgo tufting near the bottom of the headboard. Your pillows will be covering that part of the headboard anyway.

Post pics as you work on this. It would be fun to see!

You can piece together fabric in your tufts, and no-one will ever know that its pieced. Most leather tufting is pieced many times, especially headboards and sofa's.
Headboards are easier to tuft in its not a solid piece of plywood, just build a frame and stretch a piece of heavy stiff fabric across to support the foam and button tufts. A button needle is essential for this method.
Whether its diamond or square tufts, it all starts the same. Determine the size tuft you want, draw straight horizontal and vertical lines, centered and space the distance evenly on headboard. At the intersection of each horizontal and vertical lines is where you want your buttons. If you use 1 1/2" to 2" foam, bore about 1" holes in these same locations. Take you fabric and draw on the back side the same horizontal and vertical lines, but add about 1 1/4" vertical and 1" horizontal. Again mark the intersections, this is where your buttons will be placed on the fabric. Go ahead and place all the buttons: string your buttons, using a button needle, pull through the holes and tie off on the back side. Then its just matter of folding in the folds from button to button and pulling over and stapling on the back side.

Tufting your own headbaord! Bravo. Show us some pictures when you finish.